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Question 1

What is bias

Answer 1

…any systematic error in an epidemiologic study that results in an incorrect estimate of the association between exposure and risk of disease.

Question 2

What happens when we have systematic error

Answer 2

Findings could be:
• Over-estimated
• Under-estimated
• Not affected

Question 3

When can selection and information bias be controlled?

Answer 3

• Selection and information bias can only be controlled during the design and data collection phases of a study

Question 4

Selection bias

Answer 4

“ …occurs when there is a systematic difference between the people who are included in a study and those who are not, or when study and comparison groups are selected inappropriately or using different criteria”

Question 5

Minimising loss to follow-up to reduce systematic error

Answer 5

• Alternative contact details obtained at the start of the study
• Maintaining regular contact e.g. study newsletters
• Making several attempts to contact people

Question 6

Are all study designs susceptible to bias? All to the same level?

Answer 6

“ While all analytic studies are subject to bias, each design has particular types to which it is inherently most vulnerable”

Question 7

Considering selection bias - three questions to ask

Answer 7

• Who entered the study?
• Is the sample representative of the source population?
• What is the response rate?

Question 8

S election bias: Cross-sectional studies
- three things to think about

Answer 8

• Who entered the study?
• Is the sample representative of the source population?
• What is the response rate?

Question 9

Selection bias: Case-control studies - how does it arise

Answer 9

Participants are selected on the basis of their outcome status. If this selection is in some way dependant on their
exposure status then bias can occur

Exposure and outcome have already occurred
Cases and controls are selected separately

Selection of cases and controls must be independent of the exposure status

Question 10

Selection of cases and controls must be independent of the ______

Answer 10

Selection of cases and controls must be independent of the exposure status

Question 11

case - control study - how do you reduce bias

Answer 11

Ensure the identification or participation of cases is NOT related to exposure status

• Ensure high participation
• Clearly defined population of interest
• Reliable way of ascertaining all cases or a representative sample of cases

Question 12

Potential biases in selection of controls

Answer 12

• If controls are not representative of the population which gave rise to cases (in place or time)
• If inclusion/exclusion criteria differ between cases and controls
• If non-participation of eligible controls

Hypothetically, what if its easier to find a case who has the exposure????

Question 13

Cohort studies - what is the main cause of selection bias?

Answer 13

Loss to follow up
• If loss to follow-up is related to both the exposure and the
outcome this can lead to bias

Question 14

For loss to follow up to cause bias in cohort studies, what needs to happen?

Answer 14

• If loss to follow-up is related to both the exposure and the
outcome this can lead to bias

Question 15

Two key ways selection bias occurs in cohort studies

Answer 15

• If comparison group selected separately from exposed group can lead to bias
• loss to follow up

Question 16

RCT bia?

Answer 16

Loss to follow up