Linear regression model for orang-utan abundance on an expedition, conditional on at least one detection.

Model 2a provides full model of non-zero abundance. Model 2b provides model of non-zero abundance, omitting Person. Year = year in which expedition was conducted. Person = number of people on an expedition. Log(Days) = natural logarithm of duration of expedition in days. Residual standard error in Model 2a: 0.97 on 39 degrees of freedom. Residual standard error in Model 2b: 0.9693 on 40 degrees of freedom. Significance code: ‘***’: p<0.001.