Self-Selection Bias in a Case-Control Study
Selection bias can be introduced into case-control studies with low response or participation rates if the likelihood of responding or participating is related to both the exposure and outcome.
Example:
A case-control study explored an association between family history of heart disease (exposure) and the presence of heart disease in subjects. Volunteers for "a study on heart disease" were recruited from an HMO, but what if subjects with heart disease were more likely to participate if they had a family history of heart disease? If a random selection of 1,000 members of the HMO had been invited to participate and all of them did, then the true odds ratio would have been 2.25 based on the counts in the first table below.
| True Association |
CHD |
No CHD |
|
Family History |
300 |
200 |
|
No Family History |
200 |
300 |
Therefore, the true OR= (3/2)/(2/3) = 2.25
However, suppose the response rates were only 60%, except for those with CHD and a family history, who might have been more likely to participate since they were curious about whether there was an association. Once again, the observed frequencies, shown in the next table, would not have reflected the distribution in the source population, and the resulting odds ratio would be biased.
|
Biased Association |
CHD |
No CHD |
|
Family History |
240 ( 80% ) |
120 (60%) |
|
No Family History |
120 (60%) |
180 (60%) |
The biased OR = (240/120)/(120/180) = 3.0
The only was to avoid this type of bias is to achieve high participation rates. The closer one gets to 100% participation, the less likely it is that this bias will affect the results. Participation of 80% or more of those invited is usually sufficient.
Self-selection Bias Occurs Only if Participation is Differential for a Particular Exposure-Disease Cell
If participation is different by exposure status, but not by disease status, the result is not biased. We can illustrate this using the example above in which the unbiased OR = 2.25.
|
(Not Biased) |
CHD |
No CHD |
|
Family History |
240 ( 80% ) |
160 ( 80% ) |
|
No Family History |
120 (60%) |
180 (60%) |
Here, the participation differed between the two exposure categories, but within each exposure category participation was similar for the cases and the controls. As a result, the OR=2.25 (not biased).
Similarly, if participation differs by disease status, but not by exposure status, the result will not be biased.
|
(Not Biased) |
CHD |
No CHD |
|
Family History |
240 ( 80% ) |
120 (60%) |
|
No Family History |
160 ( 80% ) |
180 (60%) |
Here, the participation differed between the two outcome categories, but within each outcome category participation was similar for the exposure groups. As a result, the OR=2.25 (not biased).
Test Yourself
Researchers conducted a case-control study to determine whether maternal use of anti-depressant medications during pregnancy was associated with a higher risk congenital heart defects in their offspring. Mothers who recently had a baby are asked to participate in the study, and they were told what the research hypothesis was.
Could there be selection bias here? Think about this before looking at the answer below.
