FIGURE 2.

Cumulative histogram of the number of hits on PubMed when using a simple search strategy of prediction models and adding external validation to this search. Search strategies are given in Appendix A. PubMed was searched from 1961 to 2019. The total number of prediction model studies retrieved was 84 032, of which 4309 were found when adding an external validation search term. The percentage of studies with external validation increased over the years; in 1990, 0.5% of published prediction studies mentioned external validation, while in 2019 this was 7%.