Estimating the effect of the file drawer problem in meta-analysis

Although meta-analysis appears to be a useful technique to verify the existence of an effect and to summarize large bodies of literature, there are problems associated with its use and interpretation. Amongst difficult ies is the ''file drawer problem.'' With this problem it is assumed that a certain percentage of studies are not published or are not available to be included in any given meta-analysis. We present a cautionary table to quantify the magnitude of this problem. The table shows that distortions exaggerating the effect size are substantial and that the exaggerations of effects are strongest when the true effect size approaches zero. A meta-analysis could be very misleading were the true effect size dose to zero.