Fig. 3.

Contribution of the individual bibliographic databases in the pool of papers. For each database, the bar on the left (blue) shows retrieved papers as an absolute number and as the percentage of the total number of retrieved papers. The bar on the right (orange) shows the relevant papers (i.e. papers included and retained) as an absolute number and as the percentage of the total number of papers retrieved from this database. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)