Introductory Biostatistics

First, we get the individual test results for all variables (Table 11.19). These
indicate that uterine irritability is the most significant variable.

Step 1:Variable uterine irritability is entered. Analysis of variables not in

the model yields the results shown in Table 11.20.

Step 2:Variable mother’s weight is entered. Analysis of variables in the

model yields Table 11.21. Neither variable is removed. Analysis of vari-

ables not in the model yields Table 11.22. No (additional) variables meet

the 0.1 level for entry into the model.

Note: An SAS program would include these instructions:

PROC PHREG DATA LOWWEIGHT;
MODEL DUMMYTIME*CASE(0) = MWEIGHT SMOKING HYPERT
UIRRIT/SELECTION = STEPWISE SLENTRY = .10 SLSTAY = .15;
STRATA = SET;

TABLE 11.19

Variable Scorew^2 pValue

Mother’s weight 3.9754 0.0462

Smoking 0.0000 1.0000

Hypertension 0.2857 0.5930

Uterine irritability 5.5556 0.0184

TABLE 11.20

Variable Scorew^2 pValue

Mother’s weight 2.9401 0.0864

Smoking 0.0027 0.9584

Hypertension 0.2857 0.5930

TABLE 11.21

Factor Coe‰cient

Standard

Error zStatistic pValue

Mother’s weight 0.0192 0.0116 1.655 0.0978
Uterine irritability 2.1410 1.1983 1.787 0.0740

TABLE 11.22

Variable Scorew^2 pValue

Smoking 0.0840 0.7720

Hypertension 0.3596 0.5487

CONDITIONAL LOGISTIC REGRESSION 425