How we did it
Having designed the best fox snare we could, based on in-house experience, we enlisted the help of 34 professional gamekeepers, who undertook to mix our snares in with their own at random, and to document in detail how each snare performed. We were able to compare capture rates for target and non-target species in ‘experimental’ versus ‘other’ snares. The statistical analysis we used took into account the fact that individual operators faced different proportions of target and non-target species, worked in different environments, had different skills, etc.
All captures were recorded, even where the animal was detained for a very brief period before getting free. Thus we were able to compare experimental and other snares with respect to the frequency of capture for different species, and how frequently they self-released after capture.
Gamekeepers in the trial also recorded the condition of captured animals, as far as this could be judged from external examination. This allowed us to determine whether the circumstances of capture - such as entanglement with nearby fixed objects - influenced the risk of externally apparent injury or death.