This example illustrates how to fit a logistic regression model with a diffuse prior in PROC MCMC. You can also use the BAYES statement in PROC GENMOD. See Chapter 40: The GENMOD Procedure.
The following statements create a SAS data set with measurements of the number of deaths, y
, among n
beetles that have been exposed to an environmental contaminant x
:
title 'Logistic Regression Model with a Diffuse Prior'; data beetles; input n y x @@; datalines; 6 0 25.7 8 2 35.9 5 2 32.9 7 7 50.4 6 0 28.3 7 2 32.3 5 1 33.2 8 3 40.9 6 0 36.5 6 1 36.5 6 6 49.6 6 3 39.8 6 4 43.6 6 1 34.1 7 1 37.4 8 2 35.2 6 6 51.3 5 3 42.5 7 0 31.3 3 2 40.6 ;
You can model the data points with a binomial distribution,

where is the success probability and links to the regression covariate through a logit transformation:

The priors on and are both diffuse normal:






These statements fit a logistic regression with PROC MCMC:
ods graphics on; proc mcmc data=beetles ntu=1000 nmc=20000 nthin=2 propcov=quanew diag=(mcse ess) outpost=beetleout seed=246810; ods select PostSummaries PostIntervals mcse ess TADpanel; parms (alpha beta) 0; prior alpha beta ~ normal(0, var = 10000); p = logistic(alpha + beta*x); model y ~ binomial(n,p); run;
The key statement in the program is the assignment to p
that calculates the probability of death. The SAS function LOGISTIC does the proper transformation. The MODEL statement specifies that the response variable, y
, is binomially distributed with parameters n
(from the input data set) and p
. The summary statistics table, interval statistics table, the Monte Carlo standard error table, and the effective sample
sizes table are shown in Output 55.3.1.
Output 55.3.1: MCMC Results
Logistic Regression Model with a Diffuse Prior 
Posterior Summaries  

Parameter  N  Mean  Standard Deviation 
Percentiles  
25%  50%  75%  
alpha  10000  11.7707  2.0997  13.1243  11.6683  10.3003 
beta  10000  0.2920  0.0542  0.2537  0.2889  0.3268 
Posterior Intervals  

Parameter  Alpha  EqualTail Interval  HPD Interval  
alpha  0.050  16.3332  7.9675  15.8822  7.6673 
beta  0.050  0.1951  0.4087  0.1901  0.4027 
Logistic Regression Model with a Diffuse Prior 
Monte Carlo Standard Errors  

Parameter  MCSE  Standard Deviation 
MCSE/SD 
alpha  0.0422  2.0997  0.0201 
beta  0.00110  0.0542  0.0203 
Effective Sample Sizes  

Parameter  ESS  Autocorrelation Time 
Efficiency 
alpha  2470.1  4.0484  0.2470 
beta  2435.4  4.1060  0.2435 
The summary statistics table shows that the sample mean of the output chain for the parameter alpha
is –11.7707. This is an estimate of the mean of the marginal posterior distribution for the intercept parameter alpha
. The estimated posterior standard deviation for alpha
is 2.0997. The two 95% credible intervals for alpha
are both negative, which indicates with very high probability that the intercept term is negative. On the other hand, you
observe a positive effect on the regression coefficient beta
. Exposure to the environment contaminant increases the probability of death.
The Monte Carlo standard errors of each parameter are significantly small relative to the posterior standard deviations. A small MCSE/SD ratio indicates that the Markov chain has stabilized and the mean estimates do not vary much over time. Note that the precision in the parameter estimates increases with the square of the MCMC sample size, so if you want to double the precision, you must quadruple the MCMC sample size.
MCMC chains do not produce independent samples. Each sample point depends on the point before it. In this case, the correlation
time estimate, read from the effective sample sizes table, is roughly 4. This means that it takes four observations from the
MCMC output to make inferences about alpha
with the same precision that you would get from using an independent sample. The effective sample size of 2470 reflects this
loss of efficiency. The coefficient beta
has similar efficiency. You can often observe that some parameters have significantly better mixing (better efficiency) than
others, even in a single Markov chain run.
Output 55.3.2: Plots for Parameters in the Logistic Regression Example
Trace plots and autocorrelation plots of the posterior samples are shown in Output 55.3.2. Convergence looks good in both parameters; there is good mixing in the trace plot and quick dropoff in the ACF plot.
One advantage of Bayesian methods is the ability to directly answer scientific questions. In this example, you might want to find out the posterior probability that the environmental contaminant increases the probability of death—that is, . This can be estimated using the following steps:
proc format; value betafmt low0 = 'beta <= 0' 0<high = 'beta > 0'; run; proc freq data=beetleout; tables beta /nocum; format beta betafmt.; run;
Output 55.3.3: Frequency Counts
Logistic Regression Model with a Diffuse Prior 
beta  Frequency  Percent 

beta > 0  10000  100.00 
All of the simulated values for are greater than zero, so the sample estimate of the posterior probability that is 100%. The evidence overwhelmingly supports the hypothesis that increased levels of the environmental contaminant increase the probability of death.
If you are interested in making inference based on any quantities that are transformations of the random variables, you can
either do it directly in PROC MCMC or by using the DATA step after you run the simulation. Transformations sometimes can make
parameter inference quite formidable using direct analytical methods, but with simulated chains, it is easy to compute chains
for any set of parameters. Suppose you are interested in the lethal dose and want to estimate the level of the covariate x
that corresponds to a probability of death, p. Abbreviate this quantity as ldp
. In other words, you want to solve the logit transformation with a fixed value p. The lethal dose is as follows:

You can obtain an estimate of any ldp
by using the posterior mean estimates for and . For example, lp95
, which corresponds to , is calculated as follows:

where –11.77 and 0.29 are the posterior mean estimates of and , respectively, and 50.79 is the estimated lethal dose that leads to a 95% death rate.
While it is easy to obtain the point estimates, it is harder to estimate other posterior quantities, such as the standard
deviation directly. However, with PROC MCMC, you can trivially get estimates of any posterior quantities of lp95
. Consider the following program in PROC MCMC:
proc mcmc data=beetles ntu=1000 nmc=20000 nthin=2 propcov=quanew outpost=beetleout seed=246810 plot=density monitor=(pi30 ld05 ld50 ld95); ods select PostSummaries PostIntervals densitypanel; parms (alpha beta) 0; begincnst; c1 = log(0.05 / 0.95); c2 = c1; endcnst; beginnodata; prior alpha beta ~ normal(0, var = 10000); pi30 = logistic(alpha + beta*30); ld05 = (c1  alpha) / beta; ld50 =  alpha / beta; ld95 = (c2  alpha) / beta; endnodata; pi = logistic(alpha + beta*x); model y ~ binomial(n,pi); run; ods graphics off;
The program estimates four additional posterior quantities. The three lpd
quantities, ld05
, ld50
, and ld95
, are the three levels of the covariate that kills 5%, 50%, and 95% of the population, respectively. The predicted probability
when the covariate x
takes the value of 30 is pi30
. The MONITOR= option selects the quantities of interest. The PLOTS= option selects kernel density plots as the only ODS graphical output, excluding the trace plot and autocorrelation plot.
Programming statements between the BEGINCNST and ENDCNST statements define two constants. These statements are executed once at the beginning of the simulation. The programming statements
between the BEGINNODATA and ENDNODATA statements evaluate the quantities of interest. The symbols, pi30
, ld05
, ld50
, and ld95
, are functions of the parameters alpha
and beta
only. Hence, they should not be processed at the observation level and should be included in the BEGINNODATA and ENDNODATA statements. Output 55.3.4 lists the posterior summary and Output 55.3.5 shows the density plots of these posterior quantities.
Output 55.3.4: PROC MCMC Results
Logistic Regression Model with a Diffuse Prior 
Posterior Summaries  

Parameter  N  Mean  Standard Deviation 
Percentiles  
25%  50%  75%  
pi30  10000  0.0524  0.0253  0.0340  0.0477  0.0662 
ld05  10000  29.9281  1.8814  28.8430  30.1727  31.2563 
ld50  10000  40.3745  0.9377  39.7271  40.3165  40.9612 
ld95  10000  50.8210  2.5353  49.0372  50.5157  52.3100 
Posterior Intervals  

Parameter  Alpha  EqualTail Interval  HPD Interval  
pi30  0.050  0.0161  0.1133  0.0109  0.1008 
ld05  0.050  25.6409  32.9660  26.2193  33.2774 
ld50  0.050  38.6706  42.3718  38.6194  42.2811 
ld95  0.050  46.7180  56.7667  46.3221  55.8774 
The posterior mean estimate of lp95
is 50.82, which is close to the estimate of 50.79 by using the posterior mean estimates of the parameters. With PROC MCMC,
in addition to the mean estimate, you can get the standard deviation, quantiles, and interval estimates at any level of significance.
From the density plots, you can see, for example, that the sample distribution for is skewed to the right, and almost all of your posterior belief concerning is concentrated in the region between zero and 0.15.
Output 55.3.5: Density Plots of Quantities of Interest in the Logistic Regression Example
It is easy to use the DATA step to calculate these quantities of interest. The following DATA step uses the simulated values
of and to create simulated values from the posterior distributions of ld05, ld50, ld95,
and :
data transout; set beetleout; pi30 = logistic(alpha + beta*30); ld05 = (log(0.05 / 0.95)  alpha) / beta; ld50 = (log(0.50 / 0.50)  alpha) / beta; ld95 = (log(0.95 / 0.05)  alpha) / beta; run;
Subsequently, you can use SAS/INSIGHT, or the UNIVARIATE, CAPABILITY, or KDE procedures to analyze the posterior sample. If you want to regenerate the default ODS graphs from PROC MCMC, see Regenerating Diagnostics Plots.