Selection models with monotone weight functions in meta analysis.

Publication bias, the fact that studies identified for inclusion in a meta analysis do not represent all studies on the topic of interest, is commonly recognized as a threat to the validity of a meta analysis. One way to explicitly model publication bias is via weighted probability distributions. We adopt the non-parametric approach initially introduced by Dear and Begg (1992) but impose that the weight function w is monotonely non-increasing as a function of the p-value. Since in meta analysis one typically only has few studies or "observations," regularization of the estimation problem seems sensible. In addition, virtually all parametric weight functions proposed so far in the literature are in fact decreasing. We discuss how to estimate a decreasing weight function in the above model and illustrate the new methodology on two well-known examples. Some basic properties of the log-likelihood function and computation of a p-value quantifying the evidence against the null hypothesis of a constant weight function are indicated. In addition, we provide an approximate selection bias adjusted profile likelihood confidence interval for the treatment effect. The corresponding software and the data sets used to illustrate it are provided as the R package selectMeta (Rufibach, 2011).