Systematic literature survey
In the compiled dataset of 505 published ecological meta-analyses, 35% used log response ratios, 36% used standardized mean differences, 24% used correlation coefficients and 5% used a combination of the three investigated effect size measures. At least 64 % of the investigated ecological meta-analyses encountered missing variance measures or sample sizes in the primary literature (Fig. 1). Most often, the exact number of incompletely reported primary studies was not stated. Forty-five percent of meta-analyses just noted that they included only completely reported primary studies. The highest percentage of missing data was reported for those studies that summarized response ratios. For 10% of the studies, we could not determine whether they were affected by missing data. Most studies simply omitted incompletely reported studies from their analyses (complete-case analysis). A minor fraction of imputed missing data and only two percent of the reviewed meta-analyses (9 out of 505) applied multiple imputations or Bayesian models to account for imputation uncertainty. The proportion of meta-analyses that omitted incompletely reported studies versus those that imputed missing data did not change with the publication year (Fig. 1).