Preventive Veterinary Medicine 96 (2010) 252–262

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Use of a Randomized Response Technique to obtain sensitiveinformation on animal disease prevalence

Paul Cross ∗, Gareth Edwards-Jones, Hussain Omed, A. Prysor WilliamsSchool of the Environment, Natural Resources and Geography, College of Natural Sciences, Bangor University, Gwynedd LL57 2UW, UK

a r t i c l e i n f o

Article history:Received 22 December 2009Received in revised form 5 May 2010Accepted 14 May 2010

Keywords:Animal healthEctoparasiticPsoroptes ovisFarm biosecurityWelfare

a b s t r a c t

In order for policy-makers to formulate effective disease control measures they requireaccurate estimates of the extent and prevalence of the disease. On occasion, obtaining thesedata can be difficult, as farmers may perceive relevant information as being sensitive todivulge. Consequently, underestimation of disease prevalence may occur due to farmerreticence about admitting to behaviours and outcomes that are considered professionallytransgressive. To overcome such weaknesses, this study used a novel social science surveymethod known as the Randomized Response Technique in order to obtain population levelestimations of transgressionary behavior and disease prevalence for the case of sheep scabin Wales. Since deregulation in 1992, sheep scab has become widespread throughout theUK. Previous estimates of sheep scab in the UK have employed survey techniques whichdo not adequately protect the anonymity of the farmers’ answers other than via verbal orinstitution backed promise. In this study, farmers at several agricultural shows in Waleswere asked to complete questionnaires according to the Randomized Response Techniqueduring the summer of 2009. The survey results suggest that 30% of farmers did not routinely

treat their sheep for scab. A further 36.5% of the surveyed farmers admitted to having sheepscab in their flocks in the past 5 years. These estimates are both higher than obtained byprevious surveys which used more traditional questionnaire structures. Survey responsesrevealed a strong willingness to reintroduce compulsory treatment measures to control or

se. Thie in th

eradicate the diseaResponse Techniqu

1. Introduction

Sheep scab is a serious animal welfare condition causedby the skin parasite Psoroptes ovis which causes an allergicdermatitis through faecal deposition on the skin of the ani-mal. P. ovis is ectoparasitic, spending its entire life-cycle onsheep (Smith et al., 2001). The acuteness of reaction to the

parasite, which ranges from mild to severe, is dependentupon the breed and age of the affected animal (Lewis, 1997).Transmission of the disease occurs primarily from sheep-to-sheep contact but can also occur through contact with

∗ Corresponding author. Fax: +44 0 1248 383642.E-mail addresses: paul.cross@bangor.ac.uk, afs202@bangor.ac.uk

(P. Cross).

0167-5877/$ – see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.prevetmed.2010.05.012

s is the first study to demonstrate the value of the Randomizede context of animal health.

© 2010 Elsevier B.V. All rights reserved.

infected areas in the environment such as rubbing againstcontaminated posts, transportation or handler’s clothing(Sargison et al., 2006a). The disease is highly contagiousand quickly spreads throughout the flock. Infected animalswill tend to scratch large areas of their fleece resulting inextensive bald patches. Careful inspection of the affectedskin lesions reveals the presence of mites (Lewis, 1997).Once the disease becomes established, the animals developpoor general condition leading to weight loss, debilitationand the birth of weak lambs which have associated highermortality rates than non-infected animals (Sargison et al.,

2006a).

Sheep scab was eradicated from the UK in 1952, beforebeing accidentally reintroduced in 1973 (Sargison et al.,2007). Deregulation and the removal of compulsory dip-ping of sheep in 1992 allowed farmers to choose their own

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P. Cross et al. / Preventive Vet

reatment regime (Smith et al., 2001). Sheep scab in Greatritain is an enforceable order (MAFF, 1997). The order laysut restrictions on the movement of infected animals andmmediate treatment where possible as a matter of urgentnimal welfare. Non-compliance with this order is a sanc-ionable activity. However, the existence of such rules aloneoes not ensure that farmers will automatically comply.

Previous studies estimate that only 40% of UK farm-rs treat their sheep prophylactically against scab in spitef the high animal welfare costs and the associated eco-omic costs of the disease (Bisdorff and Wall, 2008; Milnet al., 2007). As a result, sheep scab has become endemichroughout the UK (Milne et al., 2007; Sargison et al., 2007)nd disease control and eradication is now consideredssential by some (e.g. Rose et al., 2009; Sargison et al.,006a). A number of recent studies have recorded periodrevalences for Wales of 11% (HCC, 2008), 17% (Bisdorff etl., 2006) and 24% (Rose et al., 2009). Other studies haveeported period prevalences for Great Britain of 8.6% over1-year period (Rose et al., 2009) and 36% over a 3-yeareriod (Fraser et al., 2006).

The above studies determined recorded prevalence lev-ls of flocks with scab using questionnaires. For example,isdorff et al. (2006) used a self-completed question-aire approach to estimate a sheep scab prevalence of7%. Approximately 30% of the 3530 questionnaires sentut to farmers were completed and only 17.8% of Welsharmers responded. In another study, Fraser et al. (2006)ecorded low response rates for farmers who completeduestionnaires in the South-West of England. Subsequentecruitment was made by telephone which may have intro-uced non-response bias as respondents would have beenequired to answer sensitive questions directly to thenvestigator. A recent postal study aimed to determine therevalence of scab in Welsh sheep flocks (HCC, 2008) butnly recorded a response rate of 16.7% which suggests theossibility of non-response bias.

Obtaining accurate estimates of sensitive behaviours inpopulation is problematic for a number of reasons. Low

esponse rates can undermine the validity of a survey as itntroduces the possibility that responders are not a rep-esentative sample of the study population (Asch et al.,997; de Winter et al., 2005). One potential reason for lowesponse rates in the above studies derives from the sen-itivity of the topic, and a corresponding reluctance on theart of farmers to reveal the presence of scab in their flock.

Respondents may also be unwilling to respond to directr traceable questioning techniques about sensitive sub-ects, such as scab, due to potential sanctions or peerisapproval (Keane et al., 2008; Lee, 1993). Examples ofuch respondent evasiveness are widespread in surveys ofatural resource management such as illegal fish catch inhe fisheries industry (Agnew et al., 2009; Pitcher et al.,002) or the illegal hunting of bushmeat in developingountries (Golden, 2009; Loibooki et al., 2002).

The impact that the questioning technique can have

n the outcome estimate is highlighted by the follow-ng two studies of law breakers in Holland. In the first,

study by Junger (1989) of Dutch adolescents comparedolice reports with known adolescent law breakers’ self-eports. The study found that only 60–70% of the offenses

edicine 96 (2010) 252–262 253

were admitted by the adolescents. When self-report ques-tions focused on the most recent year, only 23% of theadolescents admitted their offenses. In another study ofconvicted welfare fraudsters in Holland, van der Heijdenet al. (2000) reported that in face-to-face interviews, only25% of those convicted admitted to having ever commit-ted welfare fraud. Other socially sensitive behaviours, bethey illegal (e.g. illicit drug use or tax evasion (Clark andDesharnais, 1998; Perikles et al., 2006)) or socially stigma-tizing (e.g. admitting to having an abortion or cheating inexams (Scheers and Dayton, 1987; Silva and Vieira, 2009))are also thought to have been under-reported, particularlyif the survey methodology involved direct techniques suchas face-to-face interviews.

Secondly, the type of survey technique used can intro-duce weaknesses to the study such as social desirabilitybias and non-response bias (Fisher, 1993; Warner, 1965).Social desirability bias arises when respondents wish topresent a favourable image of themselves through theirresponses to questions independent of the truth of theirresponses. This type of bias is more noticeable in face-to-face interviews where the desire to please the intervieweris strongest. This leads to the over-reporting of desirablebehaviours and the under-reporting of undesirable items(Bowling, 2005). Postal surveys by contrast are susceptibleto non-response bias. The reliability of the survey can beundermined if the response rate becomes too low. A typ-ically acute risk is that the non-responders may differ insome marked way from the responders. Such sample biascan invalidate attempts at population estimates (Bowling,2005; Lahaut et al., 2002). All surveys that typically seek toelicit responses using data collection techniques employingpostal, telephone, computer or face-to-face data collectionmethods are likely to suffer from non-response bias. Sur-veys that ask sensitive questions are likely to compoundlower response rates as they will be further affected bysocial desirability bias.

In the context of sheep scab, recent studies estimat-ing prevalence may have been subject to these biases. Forinstance, data collected through the use of postal surveysand combination of telephone and postal surveying havebeen subject to elevated non-response rates (Bisdorff et al.,2006; HCC, 2008; Rose et al., 2009) or social desirability bias(Fraser et al., 2006) when collecting sensitive data.

Survey tools exist to reduce the influence of socialand non-response bias. Guarantees of confidentiality andanonymity can increase response rates particularly if theinterviewer does not have access to respondents’ answers.One such tool is the Randomized Response Technique(RRT). RRT reduces the stigma of answering honestly byanonymising responses in such a way that the interviewercannot attribute sensitive behaviours to an individual.The interviewer can nonetheless estimate the proportionof the sample population engaging in transgressionarybehaviour.

This technique has been used in a broad range of studies

(Goodstadt and Gruson, 1975; Scheers and Dayton, 1987;Wimbush and Dalton, 1997), e.g. to successfully estimatetransgressionary behaviours in a variety of environmentalconservation contexts (Schill and Kline, 1995; Solomon etal., 2007; St. John et al., 2010; Wright, 1980). However, RRT

erinary

2.3. Respondent sample

Farmers were recruited to the study at a series ofagricultural shows during July and August 2009 withthe cooperation of two national farming unions; NFU

254 P. Cross et al. / Preventive Vet

has not been used in the study of sensitive farming issuessuch as regulated animal diseases.

To improve response rates and reduce evasiveresponses, the RRT was used to estimate the sheepscab period prevalence in Welsh flocks. This methodwas chosen as it has a number of advantages over moredirect methods of information elicitation techniques.Firstly, RRT becomes increasingly more valid as thestudy subject sensitivity increases (Lensvelt-Mulders etal., 2005b). Secondly, whilst the answers are meaning-less at the individual level it is still possible to explorepopulation prevalence of an activity with explanatoryvariables using logistic regression (Lensvelt-Mulders et al.,2005a).

2. Methodology

2.1. Randomized Response Technique (RRT)

To overcome concerns of respondent identification, arandomising device, such as dice or coin tossing, is intro-duced to ensure response anonymity rather than the moretypical respondent anonymity such as not asking for nameand address details. In this study, a forced response methodwas used, based upon the design developed by Boruch(1971) and applied in a study by Lensvelt-Mulders andBoeije (2007). This approach requires respondents to rolltwo dice. If the sum of the numbers face up on thedice is equal to 2, 3 or 4 then respondents are askedto answer ‘yes’ to that particular question irrespective ofthe truth of that answer. If the sum of the two face-upfigures equals 11 or 12 then the respondent is asked toanswer ‘no’, again irrespective of the truth of the state-ment. For totals of 5-10, the respondent is asked to answerthe question truthfully (either ‘yes’ or ‘no’). This processis repeated for each ‘sensitive question’ asked. Respon-dents normally roll the dice in an opaque container, likea cup, so that only they could see the results of theirrolls.

The advantage of using two dice, as opposed to onlyone (van der Heijden et al., 2000), is that the respondentbelieves that he or she has a near 50% chance of rolling aforced (2, 3, 4, 11, 12) compared to an unforced score (5,6, 7, 8, 9 & 10). The apparent near parity between forcedand unforced questions provides the respondent with anincreased sense of protection in answering honestly. Thisperception is however incorrect as in reality respondentswill roll one of the forced scores only 25% of the time. Con-sequently, the respondent has a 75% probability of rollinga number that requests an honest response to the sensitivequestion. This helps make better predictions of populationlevel prevalence and reduces statistical error (Lensvelt-Mulders et al., 2005a).

When the nature of the survey is explained and/ordemonstrated to respondents they are more predisposed to

admit to sensitive behaviours than when employing moreconventional data eliciting techniques such as face-to-faceinterviews (Goodstadt and Gruson, 1975).

Subsequent to completing the survey, transgressivebehaviour rates can be estimated using the technique

Medicine 96 (2010) 252–262

developed by Boruch (1971) whereby:

�A = � − (1 − P1)�F

P1

where �A represents the proportion of the study pop-ulation who admitted transgressive behaviour; � is theproportion of all responses that were ‘yes’; P1 representsthe probability of selecting the sensitive question (0.75);�F represents the probability of the respondent answering‘yes’ to the forced response question (0.166). The requiredsample size drawn from a population of approximately12,000 sheep farmers in Wales was 372. This is based upona 50% response distribution at the 95% confidence level. Thesample size collected (574 useable questionnaires) alloweda 4% margin of error at the 95% level for a 50% responsedistribution.

2.2. Data collection

The survey questionnaire was available in both Englishand Welsh versions and comprised three sections (seeCross et al., 2010). The first section was used to collectsocio-demographic data about the farmer (gender, age)with further questions relating to farm type (e.g. size, type,and organic) and grazing regimes (e.g. grazing of animalson the Commons or not).1 The second section comprisedthe following RRT questions, all of which required a yes/noresponse:

Q.1 Do you routinely treat your sheep against scab?Q.2 Have you had scab in your flock in the last 5 years?Q.3 If you detect scab in your flock do you always treat forit?Q.4 Do you wait for scab to appear before trying to controlit?Q.5 Would you ever send your sheep to market knowingthey have scab?Q.6 Do all of your neighbours routinely treat against scab?Q.7 Would you report your neighbour to the authorities(e.g. RSPCA, Animal Health) if you knew their sheep hadscab and weren’t being treated for it?

The final section asked farmers’ opinions regardingtreatment of scab. It is important for policy development tounderstand the reasons why farmers do not treat animalsagainst sheep scab. Participants were asked to indicate,from a pre-selected list of eleven suggested reasons, thethree most important reasons of farmers did not treat sheepfor scab (Table 1).

1 The Commons are areas of land where the principle right is to grazelivestock. In Wales these are frequently upland areas. They typically com-prise land owned by either an individual or a collective, which otherpeople have usage rights, such as livestock grazing.

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edicine 96 (2010) 252–262 255

Cymru and the FUW, and the National Sheep Associa-tion. These organisations have a presence in agriculturalshows in the form of stands, tents and pavilions, wherethe interviewer would approach farmers to take part in thestudy. This method had been used in previous studies andproved to be an effective method of gaining farmer par-ticipation (Cross et al., 2009a,b). No personal identifyinginformation was collected from farmers beyond gender,age group and county in which each respondent’s flockgrazed. This permitted respondents to complete the ques-tionnaire in complete anonymity which in turn facilitatesgreater respondent honesty. Such a level of anonymity canalso introduce the potential for respondents to completemore than one survey. This was controlled at each agri-cultural show by research assistants being able to visuallyre-identify respondents, although if an individual movedbetween agricultural shows then multiple survey comple-tions are a possibility but unlikely.

2.4. Data analysis

Data were analysed using SPSS version 12 (SPSS Inc.,2003). Where appropriate, associations were exploredusing Spearman’s rank and Pearson’s correlations. Dif-ferences between means were considered significant atthe p < 0.05 level. Confidence intervals were calculatedfor results to the sensitive question section. This followsthe procedure used by Bisdorff et al. (2006) and calculatesthe confidence interval as (1·96

√p1q)/n, where p1 is the

proportion who answered yes, q is 1 − p1 and n is thesample size.

Data were aggregated by county, and Denbighshireand Flintshire data were combined as there were too fewrespondents from Flintshire. Equally, the counties of Mon-mouthshire, Newport, Blaenau Gwent, Rhondda and theVale of Glamorgan were combined into one group, subse-quently known as The Valleys.

3. Results

In total, 588 questionnaires were collected in Julyand August 2009 from five agricultural shows in Wales,29 of which used the Welsh language version. Four-teen respondents were excluded as they pertained tosheep farms in England, Scotland or Northern Ireland.The sample demographic consisted of 514 males and 72females with two respondents omitting their gender. Thenumber of respondents in each age-group class was as fol-lows; 18–34 = 93; 35–44 = 107; 45–54 = 163; 55–64 = 130;over 65 = 92 with three respondents omitting their age.Organic farm owners accounted for approximately 10%of the sample (organic farm = 56; conventional = 532).Respondents kept flocks in nine counties in Wales. Flocksranged in size from 1 to 5500 breeding ewes and themedian flock size for the survey was 350 breeding ewes(Fig. 1). Approximately 19% of farmers reported that they

graze their sheep on common land at some point duringthe year.

Farmers were asked to indicate their level of concernregarding a number of sheep-related diseases (Fig. 2).Farmers identified fly strike and sheep scab as the disease

256 P. Cross et al. / Preventive Veterinary Medicine 96 (2010) 252–262

Fig. 1. Survey respondent numbers and median flock size (in brackets) disaggregated by county in a study to obtain sensitive information on sheep scabprevalence in Wales.

Fig. 2. Extent of concern by farmers for different sheep-related diseases in a study to obtain sensitive information on sheep scab prevalence in the UK.Higher scores indicate greater concern.

P. Cross et al. / Preventive Veterinary Medicine 96 (2010) 252–262 257

F obtain si respons

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3

eitgpnaw

Fy

ig. 3. Reported prevalence of non-treatment of sheep scab in a study tontervals). ‘Do you routinely treat your sheep to prevent scab?’ (Negative

hat caused them most worry and scrapie the least worry.owever, some respondents stated that they did not rateluetongue a concern because they had already vaccinatedgainst it. Most farmers considered themselves either veryonfident (53%) or fairly confident (39%) of being able todentify sheep scab. Only 4% considered themselves not atll confident.

.1. Causes of non-treatment

It is apparent that a potential cause of elevated scab lev-ls in Wales is the use of Commons for grazing. Almost onen five (19%) of the surveyed farmers grazed their flocks onhe Commons at some point during the year. Flocks that

razed on common land had a significantly higher periodrevalence of scab (60.7%) compared to flocks that didot graze on common land (41.5%) (p < 0.001). There waslso a strong and significant correlation between farmersho grazed their sheep on Commons at some point during

ig. 4. Recorded prevalence of sheep scab in a study to obtain sensitive informatou had scab in your flock in the last five years?’ (Positive responses reported).

ensitive information on sheep scab prevalence in the UK (95% confidencees reported).

the year and the period prevalence of scab in their flocks(r = 0.17, p < 0.001).

The most common causes chosen to explain whyfarmers don’t treat for scab were ‘cost of treatment andlabour is too high’ (52.5%); ‘human health concerns of usingorganophosphates’ (51%) and a ‘lack of concern/awarenessof the disease’ (36%) (Table 1). The least likely causes were‘imminent sale of sheep’ (6%) and ‘advised to treat only whennecessary because of possible resistance developing for wormtreatment’ (7%).

Almost two thirds (63%) of respondents reported dip-ping as a preferred method for controlling scab. Sevenfarmers voiced a preference for compulsory treatment, butnot necessarily dipping.

3.2. Sensitive questions

In response to the question ‘Do you routinely treat yoursheep against scab?’ it is estimated that 29.4% of the sample

ion on sheep scab prevalence in the UK (95% confidence intervals). ‘Have

258 P. Cross et al. / Preventive Veterinary Medicine 96 (2010) 252–262

t of sheeou alwa

Fig. 5. Recorded prevalence of transgressionary behaviour in the treatmenin the UK (95% confidence intervals). ‘If you detect scab in your flock do y

did not routinely treat for scab. There were significant dif-ferences (p < 0.01) in the levels of non-treatment betweenwestern counties such as Carmarthenshire and Ceredigion(approximately 50% treatment) and northern counties suchas Gwynedd, Anglesey and Conwy (approximately 80% orhigher) (Fig. 3).

Overall, 36.5% of all farmers had or have had scab in theflock in the past 5 years. There were significant differencesin the levels between counties (p = 0.04). The lowest esti-mated prevalence of scab was approximately 19% and 24%in Carmarthenshire and Ceredigion, respectively. The high-est estimated levels were in those counties reporting thehighest treatment rates, namely Gwynedd (49.5%), Angle-

sey (54.5%) and Conwy (61%) (Fig. 4).

In response to the question ‘If you detect scab in yourflock do you always treat for it?’ it is estimated that only8.5% did not always treat for scab if they detected scab

Fig. 6. Recorded prevalence of transgressionary behaviour in the transport to minformation on sheep scab prevalence in the UK (95% confidence intervals). ‘Woulresponses reported).

p scab in a study to obtain sensitive information on sheep scab prevalenceys treat for it? (Negative responses reported).

in the flock (Fig. 5). There were no significant differencesbetween counties (p = 0.61) The Valleys was −5.54% whichindicates that some farmers found this question so sensi-tive that even when instructed to write ‘yes’ on the answersheet they refused to do so. Approximately 34% of farm-ers were estimated to wait for scab to appear in their flockbefore they treated their sheep.

Very few of the farmers were estimated to be sendingsheep to market knowing they had scab and a number offarmers were not prepared to admit to this behaviour evenwhen the question methodology required them to do so.This is evidenced by the negative scores for most counties.It appears that a few farmers may be sending their sheep

to market in Conwy and possibly in The Valleys knowingthat they have scab (Fig. 6).

More than 70% of farmers thought that not all of theirneighbours were treating for scab (Fig. 7). Most counties

arket of sheep knowingly carrying scab in a study to obtain sensitived you ever send your sheep to market knowing they have scab?’ (Positive

P. Cross et al. / Preventive Veterinary Medicine 96 (2010) 252–262 259

F ting thep s routin

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4

4

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ig. 7. Recorded prevalence of neighbouring farms thought not to be trearevalence in the UK (95% confidence intervals). ‘Do all of your neighbour

eported similar proportions (within a 20% range (55–75%))f farmers who thought their neighbours might not bereating their sheep for scab. By contrast, most farmersn the Valleys (93%) thought at least one of their neigh-ours was not treating for scab. Almost 35% of respondentsould report their neighbour to the appropriate author-

ty if they knew their neighbour wasn’t treating infectedheep for scab. Farmers in the Valleys were most willingo report non-treatment (59%) whilst farmers in Powysnd Conwy were least willing to report (25% and 28%,espectively).

. Discussion

.1. Prevalence

There have been three recent attempts to estimate scabrevalence in Wales. The first by Bisdorff et al. (2006)ecorded a prevalence of 17% in 2003/04 for Wales, whilsthe second by HCC (2008) recorded a prevalence of 11.6%n 2005/06. Rose et al. (2009) resurveyed the samples reported in Bisdorff et al. (2006) and found a preva-ence of approximately 24%, although this was based on

response rate of 52% and a resultant sample size of 40ompleted questionnaires. In a study of prevalence acrossreat Britain, the overall prevalence was reported to be 35%etween 2002/05 (Fraser et al., 2006). This correspondsore closely with the overall period prevalence for this

tudy (36.6%) which also reports prevalence over multipleears.

The apparent trend of high levels of non-treatment forcab in southern and western counties of Wales comparedo those in northern counties inversely corresponds withatterns of scab prevalence in flocks for the same coun-

ies. For example, farmers in Ceredigion had higher levelsf non-treatment for scab (47.5%) than farmers in Gwynedd12%) (Fig. 3). Only 25% of Ceredigion farmers had experi-nced scab in their flock in the past 5 years compared to9.5% for Gwynedd farmers (Fig. 4).

ir sheep for scab in a study to obtain sensitive information on sheep scabely treat against scab?’ (Negative responses reported).

There is an intriguing relationship between high preva-lence of the disease and more elevated prophylactictreatment levels in the counties of North Wales. The periodprevalence of scab in Conwy (61%) was the highest of allthe counties in Wales in spite of equally high levels of pro-phylactic treatment (81%). This could be due to a numberof reasons such as ineffective disease treatment, diseasemisdiagnoses, poor timing of treatment or employing aninappropriate control method. However, Conwy and theValleys were the only area where farmers appeared toadmit to knowingly taking scab infected sheep to mar-ket and this may be another route for the spread of thedisease, with the potential for affecting large numbers offlocks. Conwy farmers were also the second least willinggroup to report neighbours to the authorities if suspectedof not treating for scab. Interestingly, a large proportionof farmers from Conwy, Flintshire and Powys consideredthe human health concerns of organophosphate use to bean important barrier to farmers treating their sheep forscab. In summary, farmers from Conwy treat their animalsmore than other counties; have higher levels of scab; somefarmers take infected animals to market; and they are lessinclined to report their neighbours.

4.2. Randomized Response Technique

The use of RRT in this study has facilitated a more accu-rate appreciation of the levels of sheep scab in Welsh flockscompared to previous methods. This is due primarily to thetechnique’s attribute of assuring response anonymity, evenif the respondent is identifiable. So, researchers may knowthe name and address of a respondent if necessary but theycannot determine whether the answer given by the respon-dent was forced or not and hence the verity of such an

answer cannot be attributed. This study has been able toidentify those counties of Wales where sheep scab infectionis particularly acute, such as Conwy. Alternative data col-lection methodologies such as self-reported postal surveyswould be expected to detect lower rates of transgression-

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260 P. Cross et al. / Preventive Vet

ary behaviour and consequently under-estimate the extentof the disease (Blank and Gavin, 2009; Lensvelt-Mulderset al., 2005b). In spite of the relative effectiveness of RRTto gather information on transgressionary behaviours, thetechnique will nonetheless tend to underestimate preva-lence as evidenced by the negative values in Fig. 6. Thequestion ‘Would you ever send your sheep to market know-ing they have scab?’ was of such sensitivity that even whenforced to respond ‘yes’ to the sensitive behaviour somerespondents still wrote ‘no’.

Other disadvantages associated with the RRT method,include the larger sample size required to overcome thelarger standard error as a result of the dual effects ofthe variation in the sample proportion who are forced toanswer ‘yes’ and the variation in the sample proportion thatanswers ‘yes’ to the sensitive question (Buchman and Tracy,1982). The larger required population sample size also hasthe effect of increasing research costs (Lensvelt-Mulders etal., 2005a).

A further caveat requires consideration when interpret-ing the results is the potentially ambiguous interpretationof the word “treat” in survey questions 1 and 6. For instance,survey questions ask the respondent if they (or their neigh-bours) treat against scab in question 1 and 6, and whetherthey treat for scab if detected in the flock in question 3.The questions do not explicitly ask respondents if theytreat prophylactically or therapeutically, a distinction pre-viously made in the study by Bisdorff and Wall (2008).During the question construction stage it was understoodby the authors that question 1 and 6 covered prophylac-tic aspects of disease treatment as they both rely upon theword “against” to indicate prophylactic treatment. Ques-tion 1 and 6 would nonetheless have benefited from amore clearly defined use of the word “treat” as a protectionagainst any confusion in respondent answers.

Questions 1, 3 and 6 also suffer from the potential weak-ness of not having a ‘not-applicable’ category. If there are nocases of sheep scab in flocks in a given area then respon-dents will wish to answer all three questions with “no”.Unfortunately, a “no” response in the survey may be inter-preted to indicate transgressive behaviour. This holds trueparticularly for questions 1 and 6. For question 3, it wasconsidered that if respondents had none or had never hadscab in their flocks then yes/no responses would still beappropriate as they would signal an intention to act or not.Further, a ‘not applicable’ category is not possible with RRTtype questions as it would remove some of the protectionof anonymised responses. For instance, as a ‘not applicable’response indicates no disease in the flock, those respon-dents who do have infected flocks may tend to choose ‘notapplicable’ to protect their response anonymity, undermin-ing one of the primary advantages of this technique.

4.3. Opinions on the prevalence of scab according tofarmers and specialists

Approximately 19% of farmers thought that misiden-tification of the disease explained why farmers did nottreat their sheep for scab. This was the eighth least impor-tant explanation chosen by farmers from a list of elevenpossible explanations. Of the 45 farmers (8%) who acknowl-

Medicine 96 (2010) 252–262

edged that they had none or only very slight confidencein being able to diagnose sheep scab, almost half (46%)continued to routinely treat their animals for scab. Over-all there was a significant correlation between farmers’level of confidence to diagnose sheep scab and their rou-tine treatment of the disease (r = 0.21, p < 0.001). The 67%of farmers who felt confident in their diagnostic ability inthis study was somewhat less than the 83% reported bythe HCC (2008). It is not possible to determine whetherthis difference in levels of confidence is a real change overtime or whether the anonymity of RRT facilitates individu-als admitting to knowledge gaps. In stakeholder interviews,both the FUW and HCC suggested misidentification was animportant contributory factor in the current levels of infec-tion. Other comments from other organisations includedfarmers becoming habituated to the disease and there-fore downgrading its importance; farmers only treatingthose animals that scratch and not the whole flock; farmersnot treating because neighbouring farmers are not treatingtheir flocks; and disposal of dip is prohibitively expensive.

4.4. Scab control

Another potential cause of elevated scab levels inWales is the use of the Commons for grazing. There areapproximately 175,000 hectares of Common land in Wales,covering about 8.4% of the country (WAG, 2009). Concernswere voiced at the Welsh Assembly Sheep Scab Consulta-tion Workshops in 2009 that Commons are perceived asareas of risk of transmission and maintenance of sheepscab (WAG, 2009), which was confirmed by this study. Thisfinding reinforces the opinions held by the farming unionswho suggest that current sheep scab levels are in part dueto extensive grazing on common lands. This study foundthat almost two thirds of farmers (63%) thought that com-pulsory dipping should be reintroduced with a number ofother farmers opting for a broader compulsory treatment.This compares to 74% of farmers in the study by the HCC(2008). This study demonsts that scab is widespread andnot just an issue of repeat infection on the same farms butoccurs widely in a number of different regions and on amajority of farms (60%) in some counties.

4.5. Policy implications

This research informs policy-makers of the scale andextent of sheep scab in Wales. In particular, attention isdrawn to three important factors when considering futureactions at a governmental level to control scab.

Firstly, there was a marked and significant difference inthe levels of scab in flocks in counties in the North and Eastof the country. Any government programme that attemptsto control or eradicate the disease will need to consider inwhich region it is best to begin. The period prevalence inthe counties of Anglesey, Gwynedd and Conwy appears tobe sufficiently high that treatment in these counties should

be considered a priority.

Secondly, the increased likelihood of scab in flocks graz-ing on the Commons merits further attention. For instance,control measures such as the installation of physical bar-riers on common land and the treatment of all sheep

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rior to moving them to the Commons requires furtheronsideration.

Thirdly, a majority of farmers in this and other studiese.g. HCC, 2008) call for the reintroduction of compulsoryreatment. This is supported by the conclusions of a numberf other studies. For instance, Sargison et al. (2006b) argueshat voluntary scab control programmes are destined to failue to a several reasons such as identifying each and everyheep farmer in a region and the development of resistancen mites to pyrethroid and organophosphate treatments.hese ideas are supported in a study by Rose et al. (2009)ho cites the lack of compulsory measures in reporting and

reating the disease as significant barriers to the successfulontrol of the disease.

Finally, future attempts to estimate disease preva-ence from questionnaire collection will need to carefullyonsider the impact of the data collection technique onrevalence estimates. This is the first known study to adopthe RRT method to improve estimation accuracy of therevalence of a disease. It is a particularly useful tool for

nvestigating sensitive behaviours of actors in the foodhain such as farmers where sensitive questions may leado higher levels of evasive responses. Increased levels ofonest reporting will lead to more accurate estimates ofhe scale of problems such as animal welfare. This in turnill allow policy makers to better formulate policy and

mplement proportionate control interventions.

. Conclusion

This research has measured the reported prevalence ofheep scab on Welsh farms through the use of a novel ques-ionnaire technique. The Randomized Response Techniqueecorded prevalence of sheep scab in Welsh flocks of over0% over a 5-year period rising up to 60% at the individualounty level. Owing to their sensitivity, estimates of preva-ence of under-reported animal diseases such as sheep scabhould improve through the use of RRT. This would bexpected to facilitate both decision making and resourcellocation towards disease control at the policy level.

cknowledgements

The authors would like to thank the National Farmersnion Cymru, the Farmers Union of Wales, the Nationalheep Association, Animal Health, and Hybu Cig Cymruor their help in facilitating data collection. This researchas undertaken in association with the Office of the Chiefeterinary Officer for the Welsh Assembly Government.

eferences

gnew, D.J., Pearce, J., Pramod, G., Peatman, T., Watson, R., Beddington, J.R.,Pitcher, T.J., 2009. Estimating the worldwide extent of illegal fishing.PLoS ONE 4, e4570.

sch, D.A., Jedrziewski, M.K., Christakis, N.A., 1997. Response rates to

mail surveys published in medical journals. J. Clin. Epidemiol. 50,1129–1136.

isdorff, B., Wall, R., 2008. Control and management of sheep mange andpediculosis in Great Britain. Vet. Parasitol. 155, 120–126.

isdorff, B., Wall, R., Milnes, A., 2006. Prevalence and regional distributionof scab, lice and blowfly strike in Great Britain. Vet. Rec. 158, 749–752.

edicine 96 (2010) 252–262 261

Blank, S.G., Gavin, M.C., 2009. The randomized response technique as atool for estimating non-compliance rates in fisheries: a case study ofillegal red abalone (Haliotis rufescens) fishing in Northern California.Environ. Conserv. 36, 112–119.

Boruch, R.F., 1971. Assuring confidentiality of responses in social research:a note on strategies. Amer. Sociol. 6, 308–311.

Bowling, A., 2005. Mode of questionnaire administration can have seriouseffects on data quality. J. Public Health. 27, 281–291.

Buchman, T.A., Tracy, J.A., 1982. Obtaining responses to sensitivequestions: conventional questionnaire versus randomized responsetechnique. J. Account. Res. 20, 263–271.

Clark, S.J., Desharnais, R.A., 1998. Honest answers to embarrassing ques-tions: detecting cheating in the randomized response model. PsychollMethods 3, 160–168.

Cross, P., Edwards-Jones, G., Omed, H., Williams, A.P., 2010. The preva-lence of non-treatment of sheep scab in Wales and the economiccost to the livestock industry. Welsh Assembly Government, Cardiff.http://new.wales.gov.uk/topics/environmentcountryside/ahw/disease/sheepscabpi/?lang=en.

Cross, P., Williams, P., Edwards-Jones, G., 2009a. Communication of blue-tongue mitigation techniques. Vet. Rec. 165, 636–1636.

Cross, P., Williams, P., Edwards-Jones, G., 2009b. Differences in per-ceptions of farmers and veterinary surgeons about the efficacyof mitigation strategies for controlling Bluetongue. Vet. Rec. 165,397–403.

de Winter, A., Oldehinkel, A., Veenstra, R., Brunnekreef, J., Verhulst, F.,Ormel, J., 2005. Evaluation of non-response bias in mental healthdeterminants and outcomes in a large sample of pre-adolescents. Eur.J. Epidemiol. 20, 173–181.

Fisher, R.J., 1993. Social desirability bias and the validity of indirect ques-tioning. J. Consum. Res. 20, 303–315.

Fraser, D.E., Hunt, P.J., Skinner, R.J., Coles, G.C., 2006. Survey of parasitecontrol on sheep farms in south-west England. Vet. Rec. 158, 55-.

Golden, C.D., 2009. Bushmeat hunting and use in the Makira Forest, north-eastern Madagascar: a conservation and livelihoods issue. Oryx 43,386–392.

Goodstadt, M.S., Gruson, V., 1975. The randomized response technique: atest on drug use. J. Am. Stat. Assoc. 70, 814–818.

HCC, 2008. The Influence of Ectoparasites on Welsh Sheep Farms. WelshAssembly Government, Farming Connect, Hybu Cig Cymru.

Junger, M., 1989. Discrepencies between police and self-report data forDutch racial minorities. Brit. J. Crim. 29, 273–284.

Keane, A., Jones, J.P.G., Edwards-Jones, G., Milner-Gulland, E.J., 2008. Thesleeping policeman: understanding issues of enforcement and com-pliance in conservation. Anim. Conserv. 11, 75–82.

Lahaut, V.M.H.C.J., Jansen, H.A.M., van de Mheen, D., Garretsen, H.F.L.,2002. Non-response bias in a sample survey on alcohol consumption.Alcohol Alcoholism. 37, 256–260.

Lee, R.M., 1993. Doing Research on Sensitive Topics. Sage, Newbury ParkCA.

Lensvelt-Mulders, G.J.L.M., Boeije, H.R., 2007. Evaluating compliance witha computer assisted randomized response technique: a qualitativestudy into the origins of lying and cheating. Comput. Hum. Behav. 23,591–608.

Lensvelt-Mulders, G.J.L.M., Hox, J.J., Heijden, P.G.M. v.d., 2005a. How toimprove the efficiency of randomised response designs. Qual. Quant.39, 253–265.

Lensvelt-Mulders, G.J.L.M., Hox, J.J., van der Heijden, P.G.M., Maas, C.J.M.,2005b. Meta-analysis of randomized response research: thirty-fiveyears of validation. Sociol Method Res. 33, 319–348.

Lewis, C., 1997. Update on sheep scab. In Pract. 19, 558–564.Loibooki, M., Hofer, H., Campbell, K.L.I., East, M.L., 2002. Bushmeat hunting

by communities adjacent to the Serengeti National Park, Tanzania: theimportance of livestock ownership and alternative sources of proteinand income. Environ. Conserv. 29, 391–398.

MAFF, 1997. The Sheep Scab Order 1997. F.a.F. Ministry of Agriculture,editor. Surbiton, Surrey.

Milne, C.E., Dalton, G.E., Stott, A.W., 2007. Integrated control strategies forectoparasites in Scottish sheep flocks. Livest Sci. 106, 243–253.

Perikles, S., Striegel, H., Aust, F., Dietz, K., Ulrich, R., 2006. Doping in fit-ness sports: estimated number of unreported cases and individualprobability of doping. Addiction 101, 1640–1644.

Pitcher, T.J., Watson, R., Forrest, R., Valtysson, H.ó., Guénette, S., 2002.

Estimating illegal and unreported catches from marine ecosystems:a basis for change. Fish Fish. 3, 317–339.

Rose, H., Learmount, J., Taylor, M., Wall, R., 2009. Mapping risk foci forendemic sheep scab. Vet. Parasitol. 165, 112–118.

Sargison, N., Dun, K., Taylor, D., 2006a. Sheep scab control in UK flocks.Vet. Rec. 158, 309.

erinary

Warner, S.L., 1965. Randomized response: a survey technique for elimi-

262 P. Cross et al. / Preventive Vet

Sargison, N., Roger, P., Stubbings, L., Baber, P., Morris, P., 2007. Controllingsheep scab by eradication. Vet. Rec. 160, 491-b-492.

Sargison, N., Taylor, D., Dun, K., 2006b. Regional control of sheep scab inUK flocks. In Pract. 28, 62–69.

Scheers, N.J., Dayton, C.M., 1987. Improved estimation of academic cheat-ing behavior using the randomized response technique. Res. HighEduc. 26, 61–69.

Schill, D.I., Kline, P.A., 1995. Use of random response to estimate anglernoncompliance with fishing regulations. N. Am. J. Fish Manage. 15,721–731.

Silva, R.d.S.e., Vieira, E.M., 2009. Frequency and characteristics of inducedabortion among married and single women in São Paulo, Brazil. CadSaúde Pública 25, 179–187.

Smith, W.D., Van Den Broek, A., Huntley, J., Pettit, D., Machell, J., Miller,H.R.P., Bates, P., Taylor, M., 2001. Approaches to vaccines for Psoroptesovis (sheep scab). Res. Vet. Sci. 70, 87–91.

Solomon, J., Jacobson, S., Wald, K., Gavin, M., 2007. Estimating illegalresource use at a Ugandan Park with the randomized response tech-nique. Hum. Dimen. Wildl. 12, 75–88.

Medicine 96 (2010) 252–262

SPSS Inc., 2003. SPSS for Windows. SPSS Inc, Chicago.St. John, F.A.V., Edwards-Jones, G., Gibbons, J.M., Jones, J.P.G., 2010. Test-

ing novel methods for assessing rule breaking in conservation. Biol.Conserv. 143, 1025–1030.

van der Heijden, P.G.M., van Gils, G.E.R., Bouts, J.A.N., Hox, J.J., 2000. A com-parison of randomized response, computer-assisted self-interview,and face-to-face direct questioning: eliciting sensitive information inthe context of welfare and unemployment benefit. Sociol Method Res.28, 505–537.

WAG, 2009. Sheep Scab Consultation Workshops: Summary of Find-ings. Welsh Assembly Government, http://wales.gov.uk/docs/drah/publications/090408sheepscabworkshopsen.doc.

nating evasive answer bias. J. Am. Stat. Assoc. 60, 63–69.Wimbush, J.C., Dalton, D.R., 1997. Base rate for employee theft: conver-

gence of multiple methods. J. Appl. Psychol. 82, 756–763.Wright, V.L., 1980. Use of randomized response technique to estimate deer

poaching. Wildlife Soc B 8, 342–344.