Here we will go through a simple hierarchical model of species occupancy that accounts for imperfect detectability of a species. The main aims are (1) to show a variant of **binomial-family regression**, (2) to demonstrate hierarchical models with **observation and process components**, and (3) to introduce the concept of **latent variables**.

We will work with **Marc Kery’s data from chapter 20** of his Introduction to WinBUGS for ecologists. The data describe detections of *Gentianella germanica* at 150 sites, each visited 3 times. We wish to estimate the relationship between \(P\) of occurence, and a humidity index. However, we also know that the detectability of the species depends on humidity itself.