Fig. 3.

Bars show the number of studies in which the mentioned training paradigms were used (left y-axis). The scatter dot plot represents the approximate duration of training (weeks; right y-axis) per study and per training approach. Please note that the training duration is only an approximation due to partly unclear statements across the studies. The median of training duration is represented by solid horizontal lines. Open circles represent training duration in studies where dogs had previous COVID-19 detection experience. Some studies did not report training duration and were excluded from the graph (n = 4). Please note that one study used line-up and Detection Dog Training System (DDTS) approaches [45].