Fig. 5

(Silhouette plot) helps with the interpretation of how many groups are needed as well as how ‘similar’ the studies are within each group. This figure shows that the two groups get a considerable number of studies instead of one group accounting for just a handful of studies. At the same time it shows that the 10.1186/s13063-015-1107-1 ‘similarity’ within groups is relatively poor with averages within group of (0.31 and 0.22). In case of adequate discrimination and grouping, these averages would be expected in the region of 0.6. The Figure presented here was typical of that found for larger values for the number of groups