Selection Bias

Question 1

What are the 3 main selection bias that occur?

Answer 1

1. Participants are drawn form a sampling frame that is not representative of the general population (external validity)
2. Groups being compared are not from the same population (internal validity)
3. One of the groups being studied is not representative of the sampling frame from which it was drawn (internal validity)

Question 2

Give an example of a survey where there might be selection bias in which the participants are from a sampling frame that is not representative of the population?

Answer 2

lifestyle survey in sexual health clinic pts

Question 3

Give an example of a cohort study where there might be selection bias in which the participants are from a sampling frame that is not representative of the population?

Answer 3

Effect of socio-economic status on incidence of heart disease among gym members

Question 4

Give an example of a case-control study where there might be selection bias in which the participants are from a sampling frame that is not representative of the population?

Answer 4

Association of rheumatoid arthritis with heart disease among hospital pts

Question 5

Give an example of where there might be selection bias in which the groups compared are not from the same population

Answer 5

Cohort study: company incidence of a disease in an occupational group with the general population rather than with another occupational group

Case control study: a study comparing healthy GP pts with the diabetic hospital pts rather than with the diabetic GP pts

Question 6

Give an example of where there might be selection bias in which the participants are not representative of the sampling frame from which they were taken

Answer 6

Surveys: those who do respond to a survey with a low response rate are unlikely to representative of the original invitees

Cohort study: loss of follow-up