The data set docvisit
contains information for approximately 5,000 Australian individuals about the number and possible determinants of doctor
visits that were made during a twoweek interval. This data set contains a subset of variables taken from the Racd3
data set used by Cameron and Trivedi (1998). The docvisit
data set can be found in the SAS/ETS Sample Library.
The variable doctorco
represents doctor visits. Additional variables in the data set that you want to evaluate as determinants of doctor visits
include sex
(coded 0=male, 1=female), age
(age in years divided by 100), illness
(number of illnesses during the twoweek interval, with five or more coded as five), income
(annual income in Australian dollars divided by 1,000), and hscore
(a general health questionnaire score, where a high score indicates bad health). Summary statistics for these variables are
computed in the following statements and presented in Output 11.1.1.
proc means data=docvisit; var doctorco sex age illness income hscore; run;
Output 11.1.1: Summary Statistics
Variable  N  Mean  Std Dev  Minimum  Maximum  







The following statements fit a Poisson model to the data by using the covariates SEX, ILLNESS, INCOME, and HSCORE:
proc countreg data=docvisit; model doctorco=sex illness income hscore / dist=poisson printall; run;
In this example, the DIST= option in the MODEL statement specifies the POISSON distribution. In addition, the PRINTALL option displays the correlation and covariance matrices for the parameters, loglikelihood values, and convergence information in addition to the parameter estimates. The parameter estimates for this model are shown in Output 11.1.2.
Output 11.1.2: Parameter Estimates of Poisson Model
Parameter Estimates  

Parameter  DF  Estimate  Standard Error  t Value  Approx Pr > t 
Intercept  1  1.855552  0.074545  24.89  <.0001 
sex  1  0.235583  0.054362  4.33  <.0001 
illness  1  0.270326  0.017080  15.83  <.0001 
income  1  0.242095  0.077829  3.11  0.0019 
hscore  1  0.096313  0.009089  10.60  <.0001 
If some regressors are categorical in nature (meaning that these variables can take only a few discrete qualitative values), specify them in the CLASS statement. In this example, SEX is categorical because it takes only two values. A class variable can be numeric or character.
Consider the following extension:
proc countreg data=docvisit; class sex; model doctorco=sex illness income hscore / dist=poisson; run;
The partial output is given in Output 11.1.3.
Output 11.1.3: Parameter Estimates of Poisson Model with CLASS statement
Parameter Estimates  

Parameter  DF  Estimate  Standard Error  t Value  Approx Pr > t 
Intercept  1  1.619969  0.063985  25.32  <.0001 
sex 0  1  0.235583  0.054362  4.33  <.0001 
sex 1  0  0  .  .  . 
illness  1  0.270326  0.017080  15.83  <.0001 
income  1  0.242095  0.077829  3.11  0.0019 
hscore  1  0.096313  0.009089  10.60  <.0001 
If the CLASS statement is present, the COUNTREG procedure creates as many indicator or dummy variables as there are categories
in a class variable and uses them as independent variables. In order to avoid collinearity with the intercept, the lastcreated
dummy variable is assigned a zero coefficient by default. This means that only the dummy variable associated with the first
level of sex (male=0) is used as a regressor. Consequently, the estimated coefficient for this dummy variable is the negative
of the one for the original SEX
variable in Output 11.1.2 because the reference level has switched from male to female.
Now consider a more practical task. The previous example implicitly assumed that each additional illness during the twoweek interval has the same effect. In other words, this variable was thought of as a continuous variable. But this variable has only six values, and it is quite possible that the number of illnesses has a nonlinear effect on doctor visits. In order to check this conjecture, the following statements specify ILLNESS in the CLASS statement so that it is represented in the model by a set of six dummy variables that can account for any type of nonlinearity.
proc countreg data=docvisit; class sex illness; model doctorco=sex illness income hscore / dist=poisson; run;
The parameter estimates are displayed in Output 11.1.4.
Output 11.1.4: Parameter Estimates of Poisson Model with CLASS statement
Parameter Estimates  

Parameter  DF  Estimate  Standard Error  t Value  Approx Pr > t 
Intercept  1  0.385930  0.088062  4.38  <.0001 
sex 0  1  0.219118  0.054190  4.04  <.0001 
sex 1  0  0  .  .  . 
illness 0  1  1.934983  0.121267  15.96  <.0001 
illness 1  1  0.698307  0.089732  7.78  <.0001 
illness 2  1  0.471100  0.090742  5.19  <.0001 
illness 3  1  0.488481  0.099127  4.93  <.0001 
illness 4  1  0.272372  0.107593  2.53  0.0114 
illness 5  0  0  .  .  . 
income  1  0.253583  0.077441  3.27  0.0011 
hscore  1  0.094590  0.009025  10.48  <.0001 
Each ILLNESS parameter in this model represents the difference between each effect of ILLNESS and ILLNESS=5. Note that these estimates for different ILLNESS categories do not increase linearly, but instead show a relatively large jump from zero illnesses to one followed by relatively smaller increases.
Suppose that you suspect that the population of individuals can be viewed as two distinct groups: a lowrisk group, consisting of individuals who never go to the doctor, and a highrisk group, consisting of individuals who do go to the doctor. You might suspect that the data have this structure both because the sample variance of DOCTORCO (0.64) exceeds its sample mean (0.30), which suggests overdispersion, and also because a large fraction of the DOCTORCO observations (80%) have the value zero. Estimating a zeroinflated model is one way to deal with overdispersion that results from excess zeros.
Suppose also that you suspect that the covariate AGE has an impact on whether an individual belongs to the lowrisk group.
For example, younger individuals might have illnesses of much lower severity when they do get sick and be less likely to visit
a doctor, all else being equal. The following statements estimate a zeroinflated Poisson regression with AGE
as a covariate in the zerogeneration process:
proc countreg data=docvisit; model doctorco=sex illness income hscore / dist=zip; zeromodel doctorco ~ age; run;
In this case, the ZEROMODEL statement that follows the MODEL statement specifies that both an intercept and the variable AGE be used to estimate the likelihood of zero doctor visits. Output 11.1.5 shows the resulting parameter estimates.
Output 11.1.5: Parameter Estimates for ZIP Model
Parameter Estimates  

Parameter  DF  Estimate  Standard Error  t Value  Approx Pr > t 
Intercept  1  1.033387  0.096973  10.66  <.0001 
sex  1  0.122511  0.062566  1.96  0.0502 
illness  1  0.237478  0.019997  11.88  <.0001 
income  1  0.143945  0.087810  1.64  0.1012 
hscore  1  0.088386  0.010043  8.80  <.0001 
Inf_Intercept  1  0.986557  0.131339  7.51  <.0001 
Inf_age  1  2.090923  0.270580  7.73  <.0001 
The estimates of the zeroinflated intercept (Inf_Intercept) and the zeroinflated regression coefficient for AGE (Inf_age) are approximately 0.99 and –2.09, respectively. Since the zeroinflation model uses a logistic link by default, you can estimate the probabilities for individuals of ages 20, 50, and 70 as follows:









That is, the estimated probability of belonging to the lowrisk group is about 0.64 for a 20yearold individual, 0.49 for a 50yearold individual, and only 0.38 for a 70yearold individual. This supports the suspicion that older individuals are more likely to have a positive number of doctor visits.
Alternative models to account for the overdispersion are the negative binomial and the zeroinflated negative binomial models, which can be fit using the DIST=NEGBIN and DIST=ZINB options, respectively.