Contents
What is selection bias in a study?
Selection bias is a distortion in a measure of association (such as a risk ratio) due to a sample selection that does not accurately reflect the target population. This biases the study when the association between a risk factor and a health outcome differs in dropouts compared with study participants.
What type of bias is selection bias?
Selection bias is the bias introduced by the selection of individuals, groups, or data for analysis in such a way that proper randomization is not achieved, thereby ensuring that the sample obtained is not representative of the population intended to be analyzed. It is sometimes referred to as the selection effect.
When does selection bias occur in a study?
Selection bias can result when the selection of subjects into a study or their likelihood of being retained in the study leads to a result that is different from what you would have gotten if you had enrolled the entire target population.
Is there a Binary sampling indicator for selection bias?
In the general population, there are many students who aren’t admitted (A=false), but we don’t see them e.g. because we’re running a study in college research lab, and recruit using fliers around campus. Our sample is selection biased. In general, you can add a binary sampling indicator to a graph like this one.
How does conditioning on a collider introduce selection bias?
Now, we’re conditioning on S=1 (they were in the study), which is a descendant of the collider at A, and so has a similar effect as conditioning on A directly: it induces statistical dependence between M and So (social skills) conditionally on S. Conditioning on a collider or a descendant of a collider introduces selection bias!
How is selection bias used in Pharmacoepidemiology studies?
Another source of selection bias in pharmacoepidemiology studies, called prevalent user bias, occurs when the exposed group is comprised of individuals who have been exposed to the pharmaceutical of interest for some period of time before the study.