Figure 1. Flow chart showing the filtering steps, resulting in the nine articles that were used for this meta-analysis.

This flow chart was made according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) (Moher et al., 2009). Four steps are considered to be crucial for the correct evaluation of different records, in order to include them into the analysis. Whereas identification step only involves the finding of records and the elimination of redundant papers, in the screening step, all records in which data was not available are removed. After the screening process, the eligibility step involves the exclusion of all records that, even with available data, were removed for multiple reasons. For the purpose of this article, those reasons are considered as filters ii and iii (see Methods: “Data Collection and Quality Filters”). Finally, the inclusion step accounts for the final number of records used for the analysis.