Their procedure was carefully documented, including the inclusion criteria and how exactly they categorized the studies. They even had two teams independently s

Their procedure was carefully documented, including the inclusion criteria and how exactly they categorized the studies. They even had two teams independently search the literature to find studies meeting the criteria: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0132382#sec006 https://journals.plos.org/plosone/article?id=10.1371/journal... Literature reviews in meta-analyses are usually conducted like this, with specified lists of search keywords and flow charts of inclusion criteria.