If you are determined to fit this using a nonlinear method, you may want to consider NLMIXED. There you can specify various distributions in the MODEL statement. However, I will warn you that NLMIXED, while very powerful and flexible, is one of the most difficult PROCs to implement correctly. Be sure to check all of the Examples in the documentation (including the two in the Getting Started section) and especially Example 82.4 Poisson-Normal Model with Count Data.
SteveDenham
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