Choosing p as a function of risk factors means that each individual has its own probability of being infected (depending on its characteristics in terms of risk factors). It is classically assumed that the logit of p is a linear function of the different risk factors: logit(p) = a 0 +ga i X i , where X i denotes the i-th risk factor value for the individual. -Step 3: Model selection process. Different models are defined by setting some coefficients (a i ) to 0. Hence, the probability of being infected only depends on the risk factors which associated coefficients are non-zero. The different models are compared (usually using an Akaïke Information Criterium, AIC) to determine which model best describes the data.