Bias

Question 1

What is selection bias ?

Answer 1

Error in assigning individuals to groups leading to differences which may influence the outcome.

Question 2

What are the types of selection bias ?

Answer 2

Sample bias - subjects are not representative of the population.
Eg-

This may be due to volunteer bias. An example of volunteer bias would be a study looking at the prevalence of Chlamydia in the student population. Students who are at risk of Chlamydia may be more, or less, likely to participate in the study.

A similar concept is - non-responder bias. If a survey on dietary habits was sent out in the post to random households it is likely that the people who didn’t respond would have poorer diets than those who did.

Question 3

What is recall bias ?

Answer 3

Difference in accuracy of the recollections retrieved by study participants, possibly due to whether they have disorder or not.
E.g. a patient with lung cancer may search their memories more thoroughly for a history of asbestos exposure than someone in the control group.
A particular problem in case-control studies.

Question 4

What is publication bias ?

Answer 4

Failure to publish results from valid studies, often as they showed a negative or uninteresting result.

Important in meta-analyses where studies showing negative results may be excluded.

Question 5

Hawthorne effect

Answer 5

Describes a group changing it’s behaviour due to the knowledge that it is being studied

Question 6

What is lead time bias

Answer 6

Occurs when two tests for a disease are compared, the new test diagnoses the disease earlier, but there is no effect on the outcome of the disease

Question 7

What is late look bias

Answer 7

Gathering information at an inappropriate time e.g. studying a fatal disease many years later when some of the patients may have died already