Double-blinded test–retest reliability assessment
The test and the re-test aimed to measure the accuracy of the training. Each dog was naïve to the samples they used during both the test and retest phases, meaning that they had never been presented before with one of those samples. However, more than one dog could be presented with the same sample from a given volunteer and dogs could sniff during the test or the re-test samples that another dog had used in training. For this reason, during these phases the three dogs were presented with a total of n. 360 unfamiliar samples (n. 300 negatives and n. 60 positives), who were collected from a total of n. 282 participants (n. 233 RT-PCR negative, n. 43 RT-PCR positive). During testing and re-testing, only the trainer was inside the room, and she remained hidden by the wooden screen (Fig. At the beginning of the test, before the dog and the trainer entered the room, one positive and five negative samples were selected and placed in the six holders by an experimenter, who left the laboratory soon after. The location of each sample was pre-determined by this experimenter in a pseudo-random order (designed using The test took place on 2 days, 1 week apart, on which every dog performed 1 session of 5 trials. For each trial, a new set of samples of volunteers was used. However, some of the samples might have been used during training by another dog. At the end of each trial, the dog was rewarded verbally regardless of whether the marking was correct or not.To assess the repeatability (or test–retest reliability)