Calculating the probability of absence using the Credible Interval Value

A common question that arises when considering the results from a well-designed sampling programme for a rare or invasive species is: ‘Sampling has failed to detect a species that could have been present, so can we calculate the probability that it truly was absent during the sampling period?’ Noting that this invokes a Bayesian view of ‘probability’, which therefore must be accepted if the question is to be answered in the affirmative, we present a method of addressing it.