(2017) han señalado recientemente que los resúmenes exageran de alguna manera lo que el trabajo incluye, y que tras la presentación de los informes del IPCC se ha detectado un efecto de arrastre sobre determinados temas que aumentan su frecuencia en suma un resumen no refleja con exactitud el tenor de los trabajos, menos la posición asignada a los autores ante las preguntas o clasificaciones propuestas, y existen modas sobre temas de investigación que pueden suponer un acúmulo de trabajos que de alguna manera sean reiterativos y quizá no viesen la luz con tanta facilidad en otro momento. Es bien conocido que un resumen es una carta de presentación en la que se suelen 'cargar las tintas' para atraer, primero, la atención de editor y revisores y, posteriormente, de potenciales lectores. We conclude that the restricted searches are likely to lead to unrepresentative samples of studies and biased estimates of true effects. We also found significant differences in mean effect sizes between indexed and non‐indexed studies for 35% of meta‐analyses, indicating high risk of bias when the searches were restricted. Further, we found significant positive log‐linear relationships between proportions of studies missed and the deviations of mean effect sizes, suggesting that as the number of studies missed increases, deviation of mean effect size is likely to expand.

Single‐platform and multiple‐platform searches missed studies in 100 and 80 of the meta‐analyses, respectively: 52 and 46 meta‐analyses provided larger‐effect estimates 32 and 28 meta‐analyses provided smaller‐effect estimates eight and four meta‐analyses provided opposite direction of estimates and two each were unable to estimate effects due to missing all studies. We used 137 published meta‐analyses, based on multiple source searches, analysing 9388 studies: 8095 sourced from commercially published articles and 1293 from grey literature and unpublished data. We compare the impact of using single and multiple bibliographic platform(s) searches vs more comprehensive searches on estimates of mean effect sizes. Such search strategies are common in environmental evidence reviews, and if risk of bias can be detected, this would provide the first empirical evidence that comprehensiveness of searches needs to be improved.

However, selective sampling of primary studies through searches exclusively using widely used bibliographic platform(s) could bias estimates of effect sizes.

Results of meta‐analyses are potentially valuable for informing environmental policy and practice decisions. 27 We then calculated a proportion (%) of studies missed by the singleplatform search and the multiple-platform search separately for each meta-analysis by dividing the number of studies missed by the number of all studies included in the original meta-analysis. Effect size differences can occur in both positive and negative directions and so where effect size differences were negative, we transformed those to positive values, and then log-transformed all the values for obtaining a log-normal distribution 30 we did not convert any effect size metrics as there were different meanings and assumptions in each metric of effect size. 29 2.6 | Effect of proportion of studies missed on effect size differences For each eligible meta-analysis (ie, each dataset for producing a mean effect size) we calculated overall mean effect size differences between: (a) studies indexed in the single platform and all studies included in the original meta-analysis (the unweighted and weighted single-platform search groups) and (b) studies indexed in the multiple platforms and all studies included in the original meta-analysis (the unweighted and weighted multiple-platform search groups) by subtracting overall mean effect size of studies indexed in the platform(s) from overall mean effect size of all studies included in the original meta-analyses.

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