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ERROR AND BIAS IN OBSERVATIONLilik Zuhriyah

20 April 2012

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Sources of error and bias

Practical issues in reducingbias and error

Assessment the consistency of

information

Effects of misclassification onstudy results

Sub Topics

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Introduction

Assessing error and bias is the first step in

the critical appraisal of a study

Error (Non-differential error) = inaccuracy

which is similar in the different groups ofsubjects being compared

Ex : bleeding, volume of breastmilk

Bias = inaccuracy which is different in sizeor direction in one of the groups under

study than in the others

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Types of Error

Random Error

Systematic Error = Bias

any systematic error in the design,conduct or analysis of study that results in

a mistaken estimate of an exposureseffect on the risk of disease (Gordis,1996)

If the result of study close to the correctvalue, it has little error.

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True value of factor

1. Random variation

2. Biological variation

True value of factor at the point of measurement

3. Performance of the instrument used to measure the factor : bias due to the

influence of the subjects being assessed or bias due to the influence of the

observers

4. Recording of the results

5. Computation of the results

Value of the variable used in the study

Sources of Error and of Bias

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VALIDITY RELIABILITY RELATIONSHIP

Valid Tidak Valid

Reliabel A : Robin Hood

scenario: Your measure

is both reliable and valid

B : Consistent

but wrong

Tidak

Reliabel

C : Seldom hit the

center of the target but,on average, you are

getting the right answer

for the group (but not

very well for individuals.

In this case, you get a

valid group estimate,

but you are inconsistent

D : Your hits

spread acrossthe target and

you are

consistently

missing the

center. In this

case, your

measure neither

reliable nor valid

Matriks sasaran tembak (Bulls eye) (Murti, 2003 p 181 & Greenberg, 1996 p. 134)

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VALIDITY RELIABILITY RELATIONSHIP

Validity and reliability are separate ideas, in fact, theyre

related to each other

If the spots are less scattered, it show less variability and

more precision (Confidence Intervals)

Confidence intervals are a numericmeasure of the range

within which such variation is likely tooccur. The 95%confidence intervals represent the range within which we

are likelyto find the underlying true treatment effect.

The greater the precision the narrower the confidence

intervals. The greater precision results from larger sample sizes

and consequent larger numberof events.

C versus A show increased random error (error)

B & D versus A & C show systematic error (bias)

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Relation of Systematic Error & Random Error to

Study Size

Systematic Error

Random Error

Study Size

E

r

r

o

r

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Sources of Bias

Selection Bias

The way the subjects have been selected

Information Bias

The way the study variables are measured

Confounding

Sourcesof bias

Some confounding factor that is not completely controlled

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Selection Bias

When the association between exposure anddisease differs for those who participate andthose who do not participate in the study

Example :

New Screening Test to detect colon Cancer

Volunteer Vs Non Volunteer

Surveillance bias Disease maybe better in the monitored

population than in the general population

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Information Bias

Bias may be introduced in the way information

is abstracted from medical, employment or

other records or from the manner in which

interviewers ask questions.

Ex : case control study of pancreaticcancer. The respondent is a family member

Non response are generally not a

representative group.

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Evaluation of Bias

The evaluation of bias is subjective :Presence of bias

Its direction and potential magnitude of

effects on the results : Positive (OR target pop > OR sourcepop) or negative (OR target pop < ORsource pop)

Towards (underestimate) or away from(overestimate) null hypothesis

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Bias in Case Control Studies

Ensuring that cases and controls are assessed by

the same observer or group of observers, and bythe same methods used under the same

circumstances

Need additional observation (recording) :

the length of time taken for examination/

interviews

the interviewers assessment of the cooperation

of the subject and the degree of difficultyexperienced with some of the key questions

Asking the subjects / interviewer whether they

aware of any relationship

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Practical Issues in Reducing Bias and Error

The main approach to minimize error and

bias :

The definition of the item to be recorded

The choice of methods of measurementThe standardization of procedures

Quality control of all aspects of data

gathering and processing

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Assessment of bias and error

ASSESSMENT OF OBSERVATION BIAS AND ERROR

What is the definition of the factor being

assessed ?

Is it the same for each group ?

Is it appropriate to the hypothesis ?

What is the same method of

assessment ?

Instrument used

Observer making the assessment

Circumstances of use

Subjects circumstances

Subjects knowledge and cooperation

Are the methods of assessment similar

for each group ?

Are the subjects, or the observers,aware of the groping of the subjects

when the assessment is made?

How accurate and reliable is the

method of assessment?

When is the observation made ?In calendar time

In relation to the hypothesis

Is it the same for each group

How are the data handled ?

Recording and coding

computation

Are the methods the same for each

group ?

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Bias, and Methods to control bias

The most important source of bias arevariation in the subjects response to themethod of assessment, and variation in theobservers response.

The general principle for minimizing bias isto ensure that the methods used are appliedin the same mannerand with the same careto all subjects

Double blind (possible in clinical trial, but nopossible in prospective design) is to reduceerror

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lilikz.fk@ub.ac.id