METHODS SUMMARY
We conducted a comprehensive literature search, compiling scientific studies
comparing organic to conventional yields that met our selection criteria. We
minimized the use of selection criteria based on judgments of study quality but
examined its influence in the categorical analysis. We collected information on
several study characteristics reported in the papers and derived characteristics of
the study site from spatial global data sets (see Supplementary Tables 1–3 for a
description of all categorical variables). We examined the difference between
organic and conventional yields with the natural logarithm of the response ratio
(the ratio between organic and conventional yields), an effect size commonly used
in meta-analyses25. To calculate the cumulative effect size we weighted each individual
observation by the inverse of the mixed-model variance. Such a categorical
meta-analysis should be used when the data have some underlying structure and
individual observations can be categorized into groups (for example, crop species
or fertilization practices)26. An effect size is considered significant if its confidence
interval does not overlap with 1 in the back-transformed response ratio. To test the
influence of categorical variables on yield effect sizes we examined between-group