Another potential issue that may mask the association observed in meta-analysis is the presence of publication bias. This is largely a result of studies that yield significant findings being more likely to be published, therefore, it is important to search for and include “nonsignificant” studies. Missing such studies in a literature search could possibly bias the meta-analysis towards a significant association when in fact it does not exist.26 The inclusion of relevant unpublished data can negate this possible problem. In the current study, the two Begg’s funnel plots had the expected funnel shape and, in addition, the Egger’s tests for publication bias were not significant (genotype frequency data, P = 0.91; allele frequency data, P = 0.89), indicating that there was little or no publication bias in our analyses that may have impacted on the ability to observe an association. Therefore, this is unlikely to have impacted on the overall conclusions drawn from the analysis.
