census and statistics department introduction to sample surveys

政府統計處Census and Statistics Department

Introduction to Sample Surveys


Sample Surveys

Sample survey has widespread applications, ranging from surveys conducted for business purposes to collection of data for use in public affairs and social studies.

In order to ensure the reliability of survey results, it is essential that great care in addition to adequate professional knowledge be applied in the planning and conduct of surveys and in the analysis of data.


Major Steps in Conductinga Sample Survey

Overall planning Design and selection of sample Design of questionnaire Collection of raw data Compilation and analysis of statistics

and dissemination of survey results


Overall Planning

Clearly defined survey objectives Clearly defined target population Clearly defined statistical standards

(definition of terms, counting rules, classification, statistical methodologies)

Respondents should be able to provide the information required.

Adequate resources such as manpower and time must be available.


Design and Selection of Sample

Sampling deals with the selection of a number of elements, i.e. a sample.

A sample can be drawn using different methods. A probability sample, with each element having a known, non-zero chance of being included, should be used as far as practicable so that scientific inference can be drawn from the survey results.



Non-probability sampling methods, such as haphazard sampling, should be avoided. Such methods are not scientific and bias usually exists in survey results.

A complete and up-to-date sampling frame should be acquired.



In case of unequal probability of selection, it is necessary to ensure that proper weighting methods are applied to survey results.

Using established statistical methodology to compute the required sample size.

Once the sampling units are selected, alterations are not allowed.


Common ProbabilitySampling Methods

Simple random sampling Systematic sampling Stratified sampling Clustering sampling Multi-stage sampling


Non-probability Sampling Methods

Selection of elements is based on subjective judgment and experience, but not a random manner

Common non-probability sampling methodsStreet interview/mall interceptQuota samplingRespondent-initiated telephone polling Internet survey


Non-Probability Sampling Methods

Why unreliable?Scientific inference cannot be drawn from th

e sample data (results are confined to describing the group of respondents, but cannot be extracted to the entire population)

Level of precision of the estimates cannot be scientifically assessed

Biases will most likely exist in the survey results


Non-Probability Sampling Methods

Why unreliable (cont’d)?No way to know how exactly the group of res

pondents is formed• In street interviews, the interviewers tend to sele

ct those “friendly faces” for interview.• In self-selecting polls, only those who have

strong views on the survey may volunteer to participate.

The results may be inaccurate and misleading, hence not of much use.


General Principles in Designing Questionnaires

Questions should be relevant to the survey objectives

Questions should be arranged in a proper order

Use screening question to enhance the flow of the questions.

Use an appropriate language Clear instructions



Questions wordings should be appropriate, specific and precise

Avoid leading questions or questions being loaded in favour of a particular response

Avoid difficult vocabulary Avoid composite and double negative que

stions Beware of memory error



“Don’t know/No opinion” should be included as appropriate

Long questionnaires are undesirable. Questionnaires should be tested on

some prospective respondents before finalized.


Collection of Raw Data

Methods of data collection for surveys include:Self-administered questionnaires by mailPersonal interviewsTelephone interviewsComputer Assisted Telephone Interviewing

(CATI)

Very often, mixed modes of data collection can be used.



An appropriate mode should be selected by carefully considering respondents' willingness to co-operate, the degree of complexity of the subject of enquiry and other relevant factors (e.g. practicability of using personal or telephone interviews).

Comparison of Different Data Collection Methods

Characteristics Personal interview

Telephone interview

Mail questionnaire

Cost High Medium LowManpower High Medium Low

Time consuming High Medium Low

Non-response/non-contact rate

Relatively lower

Medium to high

High

Suitability for scattered population

Costly Less costly Ｍ edium

Interviewer bias Yes Yes No

Response quality High Medium Low

Length of questionnaire May be longer

Preferably short

Preferably short

Design of questionnaire May be more complicated

Simple Simple

Asking embarrassing questions

Not suitable Easier Easier



Interviewers should be trained before they start working and closely supervised during fieldwork to ensure their quality of work.

Identity and information supplied by individual respondents should be kept confidential. The survey results are to be presented in the form of aggregate statistics.



Every effort should be made to achieve a high response rate (or reducing the number of non-responses).

Methods to reduce non-responses:Keep the questionnaire brief and conciseConsider rewards to respondentsBetter publicity measures (e.g. advertisement,

advance letters)Assurance of confidentiality of individual data



Methods to reduce non-responses (Cont’d):More experienced interviewers, especially

when handling refusalsVisit households in evening timeVisit at different time of different daysIncrease the number of re-visits/call backsUse self-administered questionnaires in case

of non-contact


Compilation and Analysis of Statistics

Data should be carefully and thoroughly checked before compilation .

Appropriate statistical methodology should be adopted in compiling and analysing data.


Reporting and Assessingthe Reliability of Surveys

To enable readers to make judgment on whether the findings are credible, a good survey report should include:Sponsorship of the surveyPopulation coveredSampling methodMode of data collection Time period of data collectionWording of questions



A good survey report should include (cont’d): Sample size and response rate Point estimates and confidence intervals (if

possible) Likely sources of non-sampling errors

Information supplied by individual respondents should not be disclosed.



One may access the reliability of a survey by asking the following questions :Has probability sampling methods been use

d ?Is the sample size reasonably large ?Is the questionnaire design proper ? Any lea

ding questions or wordings ?



One may access the reliability of a survey by asking the following questions (cont’d):What is the interviewing method ? Any impro

per influence by interviewers or third parties during the interview ?

Is the response rate too low ?Are the sampling errors of acceptable magnit

ude ? How about non-sampling errors and biases ?

census and statistics department introduction to sample surveys

Documents

statistics department

probability sample

sample data results

analysis of statistics

haphazard sampling

sampling units

collection of data

survey objectives questions