Selection models with monotone weight functions in meta analysis

TL;DR

This paper introduces a nonparametric, monotone decreasing weight function approach for modeling publication bias in meta analysis, providing more flexible and insightful analysis than existing parametric methods.

Contribution

It develops a new methodology for estimating decreasing weight functions in selection models, with implementation in an R package for reproducibility.

Findings

The approach offers more flexible modeling of publication bias.

It provides confidence intervals for treatment effects adjusted for bias.

The method is illustrated on well-known meta analysis examples.

Abstract

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 the results of a meta analysis. One way to explicitly model publication bias is via selection models or weighted probability distributions. We adopt the nonparametric approach initially introduced by Dear (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. The new approach potentially offers more insight in the selection process than other methods and is more flexible than parametric approaches. 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 datasets used to illustrate it are provided as the R package selectMeta. This enables full reproducibility of the results in this paper.