The SCALE= option allows you to add a dispersion parameter to the Poisson distribution. This allows you to handle data that shows more variability than the Poisson distribution allows. Note that the mean and variance of the Poisson distribution are the same, so if the variance exceeds the mean then the data are overdispersed. Evidence of overdispersion is if the "Value/DF" for Pearson or deviance in the "Criteria For Assessing Goodness Of Fit" table is much larger than 1. One way of dealing with overdispersed data is by adding the SCALE=P (or D) option. Another way is by using the GEE method by adding the REPEATED statement. See this note for more details. Note that you don't have to repeat PARAM=REF for every variable in your CLASS statement. This is easier: class var1 var2 ... / param=ref;
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