You can use the Poisson distribution to model the distribution of cell counts in a multiway contingency table. Aitkin et al. (1989) have used this method to model insurance claims data. Suppose the following hypothetical insurance claims data are classified by two factors: age group (with two levels) and car type (with three levels). The following statements create the data set:
title 'Poisson Regression'; data insure; input n c car $ age; ln = log(n); datalines; 500 42 small 0 1200 37 medium 0 100 1 large 0 400 101 small 1 500 73 medium 1 300 14 large 1 ; proc transreg data=insure design; model class(car / zero=last); id n c age ln; output out=input_insure(drop=_: Int:); run;
The variable n
represents the number of insurance policy holders and the variable c
represents the number of insurance claims. The variable car
is the type of car involved (classified into three groups), and it is coded into two levels. The variable age
is the age group of a policy holder (classified into two groups).
Assume that the number of claims c
has a Poisson probability distribution and that its mean, , is related to the factors car
and age
for observation i by












The indicator variables is associated with the jth level of the variable car
for observation i in the following way:

A similar coding applies to age
. The ’s are parameters. The logarithm of the variable n
is used as an offset—that is, a regression variable with a constant coefficient of 1 for each observation. Having the offset
constant in the model is equivalent to fitting an expanded data set with 3000 observations, each with response variable y
observed on an individual level. The log link relates the mean and the factors car
and age
.
The following statements run PROC MCMC:
proc mcmc data=input_insure outpost=insureout nmc=5000 propcov=quanew maxtune=0 seed=7; ods select PostSummaries PostIntervals; array data[4] 1 &_trgind age; array beta[4] alpha beta_car1 beta_car2 beta_age; parms alpha beta:; prior alpha beta: ~ normal(0, prec = 1e6); call mult(data, beta, mu); model c ~ poisson(exp(mu+ln)); run;
The analysis uses a relatively flat prior on all the regression coefficients, with mean at 0 and precision at . The option MAXTUNE=0 skips the tuning phase because the optimization routine (PROPCOV=QUANEW) provides good initial values and proposal covariance matrix.
There are four parameters in the model: alpha
is the intercept; beta_car1
and beta_car2
are coefficients for the class variable car
, which has three levels; and beta_age
is the coefficient for age
. The symbol mu
connects the regression model and the Poisson mean by using the log link. The MODEL statement specifies a Poisson likelihood for the response variable c
.
Posterior summary and interval statistics are shown in Output 55.5.1.
Output 55.5.1: MCMC Results
Poisson Regression 
Posterior Summaries  

Parameter  N  Mean  Standard Deviation 
Percentiles  
25%  50%  75%  
alpha  5000  2.6403  0.1344  2.7261  2.6387  2.5531 
beta_car1  5000  1.8335  0.2917  2.0243  1.8179  1.6302 
beta_car2  5000  0.6931  0.1255  0.7775  0.6867  0.6118 
beta_age  5000  1.3151  0.1386  1.2153  1.3146  1.4094 
Posterior Intervals  

Parameter  Alpha  EqualTail Interval  HPD Interval  
alpha  0.050  2.9201  2.3837  2.9133  2.3831 
beta_car1  0.050  2.4579  1.3036  2.4692  1.3336 
beta_car2  0.050  0.9462  0.4497  0.9485  0.4589 
beta_age  0.050  1.0442  1.5898  1.0387  1.5812 
To fit the same model by using PROC GENMOD, you can do the following. Note that the default normal prior on the coefficients is , the same as used in the PROC MCMC. The following statements run PROC GENMOD and create Output 55.5.2:
proc genmod data=insure; ods select PostSummaries PostIntervals; class car age(descending); model c = car age / dist=poisson link=log offset=ln; bayes seed=17 nmc=5000 coeffprior=normal; run;
To compare, posterior summary and interval statistics from PROC GENMOD are reported in Output 55.5.2, and they are very similar to PROC MCMC results in Output 55.5.1.
Output 55.5.2: PROC GENMOD Results
Poisson Regression 
Posterior Summaries  

Parameter  N  Mean  Standard Deviation 
Percentiles  
25%  50%  75%  
Intercept  5000  2.6424  0.1336  2.7334  2.6391  2.5547 
carlarge  5000  1.8040  0.2764  1.9859  1.7929  1.6101 
carmedium  5000  0.6908  0.1311  0.7797  0.6898  0.6044 
age1  5000  1.3207  0.1384  1.2264  1.3209  1.4140 
Posterior Intervals  

Parameter  Alpha  EqualTail Interval  HPD Interval  
Intercept  0.050  2.9154  2.3893  2.8997  2.3850 
carlarge  0.050  2.3668  1.2891  2.2992  1.2378 
carmedium  0.050  0.9437  0.4231  0.9434  0.4230 
age1  0.050  1.0455  1.5871  1.0266  1.5629 
Note that the descending
option in the CLASS statement reverses the sort order of the class variable age
so that the results agree with PROC MCMC. If this option is not used, the estimate for age
has a reversed sign as compared to Output 55.5.2.