Selection bias

Question 1

Definition of selection bias?

Answer 1

• Distortions that result from procedures used to select subjects and from factors that influence participation in the study
• Arises from systematic differences in association between exposure and disease in the study population relative to the source/reference population
• Also: bias resulting from conditioning on the common effects of two variables

Question 2

Selection bias has an impact on which type of validity?

Answer 2

Both internal and external

Question 3

What is conditioning?

Answer 3

examining the distribution of one variable within levels of another variable

Question 4

What are colliders?

Answer 4

Common effects (those that receive 2 arrow heads, one from treatment and one from outcome)

Question 5

What bias can arise from conditioning on a collider?

Answer 5

Collider stratification bias: induces statistical association between its causes (e.g., between treatment and outcome)

Question 6

What is immigrative selection?

Answer 6

• Selection bias that arises during enrollment or randomization

Question 7

Two potential sources of selection bias in RCTs and cohort studies during enrolment/randomization?

Answer 7

1. Inadequate allocation concealment (RCT only)

2. Self-selection (volunteer bias) or eligibility criteria (healthy worker, survival bias)

Question 8

Two potential sources of selection bias in RCTs and cohort studies during follow-up?

Answer 8

1. Differential loss to follow-up (withdrawals, competing risks, loss of contact, protocol violations, contaminations)
2. Nonresponse/missing data bias

Question 9

What is the nonresponse/missing data bias?

Answer 9

When the analysis is restricted to individuals with complete follow-up (type of selection bias)

Question 10

How does allocation concealment failure lead to selection bias?

Answer 10

Knowledge of the next assignment could lead to exclusion of certain patients based on their prognosis

Question 11

The healthy worker bias is an example of…

Answer 11

Biased sample selection due to self-selection/eligibility

Question 12

Two potential sources of selection bias in case-control studies?

Answer 12

1. Inadequate control selection (not independently of exposure)
2. Matching

Question 13

How does inadequate control selection in case-control studies result in selection bias?

Answer 13

Controls are not an unbiased sample of the exposure distribution in the study base

Question 14

How does matching in case-control studies may result in selection bias?

Answer 14

The exposure distribution in the controls will resemble that of the cases rather than the source population

Question 15

What is Berkson’s bias? What type of bias is it?

Answer 15

Selection bias that arises when hospitalized controls are used, and that 2 diseases unassociated in the population are associated among hospitalized patients
(e.g., strong association with coffee consumption and pancreatic cancer, because controls that had intestinal disorders were used, and were thus much less likely to be drinking coffee)

Question 16

Selection bias and confounding can both lead to…

Answer 16

A lack of exchangeability between exposed and non-exposed groups and limit internal validity

Question 17

Unlike confounding, selection bias can arise in…

Answer 17

Both experimental and observational studies

Question 18

Define confounding and selection bias in terms of lack of exchangeability

Answer 18

Confounding: lack of exchangeability due to common cause

Selection bias: lack of exchangeability induced by conditioning on common effects

Question 19

4 ways to limit selection bias at the design level?

Answer 19

1. Random draw from source population (reduces immigrative s.b.)
2. Concealment of allocation
3. Careful selection of controls, or using multiple control groups
4. Limit loss to follow-up and missing data (reduces emigrative s.b.)

Question 20

2 ways to adjust for selection bias at analysis level?

Answer 20

1. Inverse probability weighting

2. Sensitivity analyses

Question 21

If there is no selection bias, what is the selection ratio?

Answer 21

1