Quoting Frank H: "These relationships rarely occur in nature..." I
agree; I have seen cutpoint relationships only a handful of times in 25
years of medical work.
A better approach is to look at the data using a smoothing spline:
options(na.action=na.exclude)
fit <- coxph(Surv(time,status) ~ age + weight + pspline(x))
temp <- predict(fit, type='terms')
plot(x, temp[,3])
My primary goal has always been to learn rather than to test.
It is common to see upper or lower thresholds, but not cutpoints.
Terry T.
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