The review in question is a meta-analysis. This is a statistically rigorous study of studies, rather than a mere summary of the literature. Its authors, Matin Qaim and Wilhelm Klümper, both of Göttingen University, in Germany, went through all examinations of the agronomic and economic impacts of GM crops published in English between 1995 and March 2014. This provides a near-complete survey. Most studies of the subject have been published in English, and the widespread adoption of such crops began only in the mid-1990s.
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Ref: A Meta-Analysis of the Impacts of Genetically Modified Crops | Klümper W, Qaim M (2014) A Meta-Analysis of the Impacts of Genetically Modified Crops. PLoS ONE 9(11): e111629. doi:10.1371/journal.pone.0111629