Example (**zip.sas**), data from 195 subjects in a
prospective longitudinal survey are analyzed. Criminal offense convictions
were recorded annually beginning for boys from age 8 through
age 32. The analysis begins with Program 1 (below). The resulting output and
graphs follow. Extra zero probability in the model is specified using the IORDER statement. IORDER 1; indicates that we'll model the
probability of extra-Poisson zeros with a linear function common to all of
the groups.

Program 1: Three-group ZIP model, cubic trajectories.

Output 1:Three-group ZIP model, shared linear zero inflation probability.

Graph 1: Sample average and predicted trajectories.

**The model appears satisfactory so we'll just try adding the risk factors
without using start values.**

Program 2: Three-group ZIP model, cubic trajectories with risk factors.

Output 2: Three-group model with risk factors.

**Daring and rearing are 0/1 variables. Daring is 1 for high levels of
daring behavior observed in the subject. Rearing is 1 for high levels of poor
rearing practices by the parents. The results show that both high levels of
daring behavior and poor rearing increase the likelihood of belonging in the higher offending
groups (insufficient evidence that poor rearing is more frequent in the middle
group in this example, p=0.0658). (zip.sas)**