Studies into participation bias have examined participation trends, where it occurs, the factors affecting it and methods to try to reduce it. However, some authors only discuss participation bias at the end of the study, some acknowledge it and apply a method to try to reduce it, while others ignore it or dismiss it as negligible.
Issues of three high impact epidemiology journals were examined; 81 articles were read and reviewed for potential participation bias. Categories were used to classify the approach taken to participation bias and the results recorded.
Of the 81 articles considered, 42 (51%) were eligible and could have suffered from participation bias. It was found that 57% of these articles ignored the effects of participation bias, while 17% only considered it briefly in the discussion. Few articles (22%) attempted to reduce the participation bias, with over half of thesesome using unsuitable methods (55%).
This review highlights how participation bias is often not considered and hence the conclusions drawn from these studies may not be correct.
Some key words:
participation, bias, selection, case-control studies.