Study selection
The electronic literature search identified 3938 studies (Figure 1); search of the grey literature and reference lists identified a further 19 studies. Following import of the literature search results, 1337 studies were immediately removed as duplicates. Screening by title and abstract of 2620 studies was performed and 2529 studies were excluded as irrelevant. The full texts of 69 remaining articles were reviewed and 43 studies were excluded as shown in Figure 1. Eventually 26 studies were included in this systematic review.