week 6

Question 1

Name 2 type of errors and then their subsections that could occur in the epidemiological studies?

Answer 1

1. Random error.

2. systematic error. (confounding) and bias- selection and information bias.

Question 2

What is random error?

Answer 2

It is a chance error. May result in under or over estimation of the true value.

Mainly related to sample size. Smaller the sample size, greater the random error.

Question 3

What is systematic error?

Answer 3

Estimate of association between
exposure (E) and outcome (O)
systematically different from
truth

• Error not related to sample size

Question 4

Describe what is Bias?

Answer 4

Usually a result of study design or
conduct.

Cannot be corrected at the analysis stage.

Different study types are prone to different biases.

Question 5

What is selection bias?

Answer 5

bias related to who is included in study

sample

Question 6

What is information bias?

Answer 6

bias related to how the study variables

(exposure / outcome) are measured

Question 7

What is selection bias? (2)

Answer 7

Occurs when there is a difference in the
characteristics of those selected and those not
selected for the study…

AND this selection is associated with exposure or
outcome of interest

Question 8

What is selection bias in case control studies?

Answer 8

• occurs when the exposure of interest is
associated with the likelihood of controls (or
cases) being included in the study

Problem
• Results in biased estimate of exposure odds in
population from which sample is drawn

Question 9

What is selection bias in cohort studies?

Answer 9

• Less of a problem than in case-control studies
• Main selection bias: loss to follow-up (attrition)
• Other selection biases: choice of unexposed
group
• E.g. healthy worker effects in occupational
studies

Question 10

What is information bias ?

Answer 10

misclassification of exposure status, outcome status or both

Question 11

What are the two types of information bias?

Answer 11

1. Non-differential (random) misclassification:
   • Of exposure independent of outcome or
   • Of outcome independent of exposure
2. Differential (non-random) misclassification:
   • Of exposure dependent on outcome or
   • Of outcome dependent on exposure

Question 12

What are the two main types of information bias?

Answer 12

1. Recall (response) bias:
   • Occurs when a respondent’s recall of exposures
   varies according to whether they are a case or a
   control
2. Observer (interviewer) bias:
   • Occurs when awareness of case / control status
   influences the assessment or recording of
   exposure status

Question 13

What is recall bias?

Answer 13

Cases or controls selectively likely to remember
or forget, or disclose or not disclose the
exposure
• This may be due to, e.g.:
• “effort after meaning”
• Social desirability bias

Question 14

How to minimise recall bias?

Answer 14

Select controls from a group who similarly are
being treated for another disorder (reduces
“effort after meaning”
• Context of interview could be similar for cases
and controls
• Use more comprehensive measures of exposure
• Multiple informants
• Blinding of cases and controls to their group

Question 15

What is observer bias?

Answer 15

Non-comparable information is obtained from
cases and controls, e.g. leading questions
• For cases & controls (in case-control studies)
• For exposed & unexposed (in cohort
studies)
• Limiting observer bias:
• Blinding observer to hypothesis/caseness
(often difficult)