Dear List,
Can anyone please explain the difference between cluster() and
frailty() in a coxph? I am a bit puzzled about it. Would appreciate
any useful reference or direction.
cheers,
Ehsan
> marginal.model <- coxph(Surv(time, status) ~ rx + cluster(litter), rats)
> frailty.model <- coxph(Surv(time, status) ~ rx + frailty(litter), rats)
> marginal.model
Call:
coxph(formula = Surv(time, status) ~ rx + cluster(litter), data = rats)
coef exp(coef) se(coef) robust se z p
rx 0.905 2.47 0.318 0.303 2.99 0.0028
Likelihood ratio test=7.98 on 1 df, p=0.00474 n= 150
> frailty.model
Call:
coxph(formula = Surv(time, status) ~ rx + frailty(litter), data = rats)
coef se(coef) se2 Chisq DF p
rx 0.914 0.323 0.319 8.01 1.0 0.0046
frailty(litter) 17.69 14.4 0.2400
Iterations: 6 outer, 24 Newton-Raphson
Variance of random effect= 0.499 I-likelihood = -180.8
Degrees of freedom for terms= 1.0 14.4
Likelihood ratio test=37.6 on 15.4 df, p=0.00124 n= 150
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