Question
Consider a ten-year follow-up of a fixed cohort free of CHD at the start of follow-up. The study objective is to determine whether persons experiencing food allergies (FA) prior to the start of follow-up are at increased risk for CHD development. Suppose that a simple analysis of the study results yields the following study information.
FA
| |||
CHD
|
Yes
|
No
|
Total
|
Yes
|
49
|
286
|
335
|
No
|
1378
|
5816
|
7194
|
Total
|
1427
|
6102
|
7529
|
RR = 0.72, OR = 0.72
Which of the following statements about possible selection bias is appropriate for this study design (Choose only one answer):
If persons who had high blood pressure (a known risk factor for CHD) prior to the start of follow-up were excluded from the comparison (no allergy) group, but NOT from the FA (allergy) group, then there should be concern about selection bias that would be AWAY FROM THE NULL.
If the likelihood of being not lost to follow-up for persons with food allergies is exactly the same as the likelihood of not being lost to follow-up for persons without food allergies, then there can be no selection bias due to follow-up loss.
Bias from selective survival should be a concern in this study.
Berkson’s bias should be a concern in this study.
If persons who were free of any kind of illness (except FA) prior to start of follow-up were excluded from both the exposed group and the comparison (no allergy) group, then there should be concern about selection bias that would be TOWARDS THE NULL.
Source: Active Epi
