Chapter Contents Previous Next
 The NLMIXED Procedure

## Logistic-Normal Model with Binomial Data

This example analyzes the data from Beitler and Landis (1985), which represent results from a multi-center clinical trial investigating the effectiveness of two topical cream treatments (active drug, control) in curing an infection. For each of eight clinics, the number of trials and favorable cures are recorded for each treatment. The SAS data set is as follows.

   data infection;
input clinic t x n;
datalines;
1 1 11 36
1 0 10 37
2 1 16 20
2 0 22 32
3 1 14 19
3 0  7 19
4 1  2 16
4 0  1 17
5 1  6 17
5 0  0 12
6 1  1 11
6 0  0 10
7 1  1  5
7 0  1  9
8 1  4  6
8 0  6  7
run;


Suppose nij denotes the number of trials for the ith clinic and the jth treatment (i = 1, ... ,8 j = 0,1), and xij denotes the corresponding number of favorable cures. Then a reasonable model for the preceding data is the following logistic model with random effects:

and
The notation tj indicates the jth treatment, and the ui are assumed to be iid .

The PROC NLMIXED statements to fit this model are as follows:

   proc nlmixed data=infection;
parms beta0=-1 beta1=1 s2u=2;
eta = beta0 + beta1*t + u;
expeta = exp(eta);
p = expeta/(1+expeta);
model x ~ binomial(n,p);
random u ~ normal(0,s2u) subject=clinic;
predict eta out=eta;
estimate '1/beta1' 1/beta1;
run;


The PROC NLMIXED statement invokes the procedure, and the PARMS statement defines the parameters and their starting values. The next three statements define pij, and the MODEL statement defines the conditional distribution of xij to be binomial. The RANDOM statement defines U to be the random effect with subjects defined by the CLINIC variable.

The PREDICT statement constructs predictions for each observation in the input data set. For this example, predictions of and approximate standard errors of prediction are output to a SAS data set named ETA. These predictions include empirical Bayes estimates of the random effects ui.

The ESTIMATE statement requests an estimate of the reciprocal of .

The output for this model is as follows.

 The NLMIXED Procedure

 Specifications Data Set WORK.INFECTION Dependent Variable x Distribution for Dependent Variable Binomial Random Effects u Distribution for Random Effects Normal Subject Variable clinic Optimization Technique Dual Quasi-Newton Integration Method Adaptive Gaussian Quadrature

The "Specifications" table provides basic information about the nonlinear mixed model.

 The NLMIXED Procedure

 Dimensions Observations Used 16 Observations Not Used 0 Total Observations 16 Subjects 8 Max Obs Per Subject 2 Parameters 3 Quadrature Points 5

The "Dimensions" table provides counts of various variables. You should check this table to make sure the data set and model have been entered properly. PROC NLMIXED selects five quadrature points to achieve the default accuracy in the likelihood calculations.

 The NLMIXED Procedure

 Parameters beta0 beta1 s2u NegLogLike -1 1 2 37.5945925

The "Parameters" table lists the starting point of the optimization.

 The NLMIXED Procedure

 Iteration History Iter Calls NegLogLike Diff MaxGrad Slope 1 2 37.3622692 0.232323 2.882077 -19.3762 2 3 37.1460375 0.216232 0.921926 -0.82852 3 5 37.0300936 0.115944 0.315897 -0.59175 4 6 37.0223017 0.007792 0.01906 -0.01615 5 7 37.0222472 0.000054 0.001743 -0.00011 6 9 37.0222466 6.57E-7 0.000091 -1.28E-6 7 11 37.0222466 5.38E-10 2.078E-6 -1.1E-9

 NOTE: GCONV convergence criterion satisfied.

The "Iterations" table indicates successful convergence in seven iterations.

 The NLMIXED Procedure

 Fit Statistics -2 Log Likelihood 74.0 AIC (smaller is better) 80.0 BIC (smaller is better) 80.3 Log Likelihood -37.0 AIC (larger is better) -40.0 BIC (larger is better) -40.1

The "Fitting Information" table lists some useful statistics based on the maximized value of the log likelihood.

 The NLMIXED Procedure

 Parameter Estimates Parameter Estimate Standard Error DF t Value Pr > |t| Alpha Lower Upper Gradient beta0 -1.1974 0.5561 7 -2.15 0.0683 0.05 -2.5123 0.1175 -3.1E-7 beta1 0.7385 0.3004 7 2.46 0.0436 0.05 0.02806 1.4488 -2.08E-6 s2u 1.9591 1.1903 7 1.65 0.1438 0.05 -0.8554 4.7736 -2.48E-7

The "Parameter Estimates" table indicates marginal significance of the two fixed-effects parameters. The positive value of the estimate of indicates that the treatment significantly increases the chance of a favorable cure.

 The NLMIXED Procedure

 Additional Estimates Label Estimate Standard Error DF t Value Pr > |t| Alpha Lower Upper 1/beta1 1.3542 0.5509 7 2.46 0.0436 0.05 0.05146 2.6569

The "Additional Estimates" table displays results from the ESTIMATE statement. The estimate of equals 1/0.7385 = 1.3541 and its standard error equals 0.3004/0.73852 = 0.5509 by the delta method (Billingsley 1986). Note this particular approximation produces a t-statistic identical to that for the estimate of .

Not shown is the ETA data set, which contains the original 16 observations and predictions of the .

 Chapter Contents Previous Next Top