Matvec Users’ Guide

76 CHAPTER 11. GENERALIZED LINEAR MIXED MODEL ANALYSES
.
B 13 1
B 17 1
B 20 1
The third column contains a code to indicate if the data are right censored (1:uncensored and 0:censored).
Because none of the records were censored, all the censor codes are one in this example. Like the threshold
model, the Weibull is a multivariate generalized linear model. Model equations will be needed for both a
risk factor η and a rate parameter ρ. Typically the rate parameter, which describes the shape of the survival
function, is assumed to be the same for everyone in the population and only includes an intercept term. The
risk factor will include both an intercept and an effect for drug therapy.
The Weibull survival model for this example can be obtained from:
D=Data();
D.input("cancer.dat","therapy $ week censor");
M=Model();
M.equation("week=intercept therapy,week=intercept");
M.link("weibull",0)
M.param(0,"week","censor",.5);
M.init(-1,1);
M.fitdata(D);
M.glim();
M.save("cancer.out");
The M.param() is used to specify four additional model fitting parameters needed by the weibull link
function. The first parameter is 1 if the rate parameter is to remain fixed and 0 if the rate parameter is to
be estimated. The second parameter, is the time variable in the data set. The third parameter, is the censor
code variable in the data set. The fourth parameter is the time to recode any zero times to in the data set.
The M.init() is used to specify initial values for both the risk factor and the rate parameter. If M.init()
was not included the initial values for the risk factor and rate factor would be zero.
A Wald test for testing for a therapy effect is generated by:
"\n Therapy effect\n"
M.contrast("therapy",[1,0,-1,0]);
To test the hypothesis that the rate parameter was 1 is generated by:
"\n Rate parameter\n"
M.contrast("intercept",[0 1],[1]);
The output from the program is
Weibull Link Function
Parameters: FixRho (scalar), Time variable (string),
Censor Variable (string) 1=uncen 0=cen,
Zero Value (scalar)
Terms: Eta, Rho
Therapy effect
RESULTS FROM CONTRAST(S)
----------------------------------------------------------
Contrast MME_addr K_coef Raw_data_code
---------------------------------------------
1 3 1 week:therapy:A