'''

1. coxph(formula = Surv(time, hyper) ~ age + drink + smoke + sodium * tchl * BMI, data = male)
2. coxph(formula = Surv(time, hyper) ~ age + drink + smoke + sodium * tchl * BMI, data = female)

• The 3-term variables are factors (=categorical)

'''

 
The result of 3 terms interaction of 1 (male, n=1660)) : 

                         coef  exp(coef)   se(coef)      z     Pr(>|z|)
 sodium:tchl:BMI        NA         NA    0.000e+00     NA       NA 
The result of 3 terms interaction of 2 (female, n=1900) : 

<&nbsp>                 coef     exp(coef)   se(coef)     z    Pr(>|z|)
 sodium:tchl:BMI       4.545e-01  1.575e+00  1.582e+00  0.287    0.774  

Why is the result of the male NA?

'''

1. coxph(formula = Surv(time, hyper) ~ age + drink + smoke + sodium * tchl * BMI, data = male)
2. coxph(formula = Surv(time, hyper) ~ age + drink + smoke + sodium * tchl * BMI, data = female)

• The 3-term variables are factors (=categorical)
• male patient:1506, control: 162
• female patient:1548, control: 363
• time: 7 periods

'''

 
The result of 3 terms interaction of 1 (male, n=1668)) : 

                         coef  exp(coef)   se(coef)      z     Pr(>|z|)
 sodium:tchl:BMI        NA         NA    0.000e+00     NA       NA 
The result of 3 terms interaction of 2 (female, n=1971) : 

<&nbsp>                 coef     exp(coef)   se(coef)     z    Pr(>|z|)
 sodium:tchl:BMI       4.545e-01  1.575e+00  1.582e+00  0.287    0.774  

Why is the result of the male NA?