Meta-analysis: Unresolved issues and future developments

Abstract

Publication bias, discussed in detail in a previous article,11 is a major threat to the validity of meta-analysis. Obtaining and including data from unpublished studies seems to be the obvious way of avoiding this problem. Including data from unpublished studies can itself introduce bias, however. Even after extensive consultation with the research community, unpublished studies may remain hidden. The unpublished studies that can be located may thus be an unrepresentative sample of unpublished studies. Whether bias is reduced or increased by including unpublished studies cannot formally be assessed as it is impossible to be certain that all unpublished studies have been located. A further problem relates to the willingness of investigators of located unpublished studies to provide data. This may depend on the findings of the study, more favourable results being provided more readily. This could again bias the findings of a meta-analysis.