The Health Worker Effect
The general population is sometimes used as a non-exposed comparison group in occupational studies of mortality, since data is readily available, and the general population is mostly unexposed to rare occupational exposures. The main disadvantage is that an employed work force tends to be generally healthier than the general population, because the general population also includes people who cannot work due to disease or disability. Consequently, the risk ratio for death will tend to be underestimated, when the higher risk general population is used as a surrogate for an unexposed work force; this is called "the healthy worker effect". Here, the source population is an employed work force, and death rates in the general population are not necessarily indicative of death rates in an unexposed work force.
Suppose an employed work force with a particular occupational exposure has an age-adjusted death rate of 130 per 100,000 workers, and the age-adjusted death rate in a very comparable employed work force that was unexposed was 100 per 100,000. In other words, the true risk ratio is 1.3.
However, the age-adjusted death rate in the general population (which is mostly unexposed to rare occupational exposures) would tend to be higher than that of an unexposed work force, perhaps 120 per 100,000 because of a wide variety of illnesses and disabilities. Using the general population as surrogate for an unexposed work force would result in a risk ratio of 130/120 = 1.08, i.e., an underestimate of the true magnitude of association.
Summary of Selection Bias
Selection bias occurs when selection, enrollment, or retention of subjects is differential for one of the exposure-disease categories. This can occur in any of the analytic studies, and it can cause either an overestimate or an underestimate of the measure of association.
In case-control studies selection bias can occur due to:
- Control selection bias: Selection of a comparison group ("controls") that is not representative of the population that produced the cases in a case-control study.
- Self-selection bias: Refusal, non-response, or agreement to participate that is related to the exposure and disease status.
In cohort studies selection bias can occur due to:
- The "healthy worker" effect, which tends to cause an underestimate of the association, since death rates in the general population are usually higher than in an employed work force, i.e., the general population is not representative of the source population.
- Differential loss to follow up can cause an overestimate or an underestimate of effect in retrospective and prospective cohort studies and in clinical trials.