Re: Cox Proportionmal Hazard Model


So this is stright forward, it's right censored data, with a continuous
independent variable. I plug my numbers inot SPSS and it says the
p-value is 0.021. So that means it is an indicator of completion time
yes? The point where I get confused is I also have their grade for
their programming class, if I use that as a variable then it too is a
significant indicator. However, and this is where I do the Cox
analysis for both of them together then it says that the self-rating
_isn't_ a significant indicator but the grade is.

Could someone explain (or point me to the explination) of what's going
on and what that means I can say about the subject's self-rating?


You could say that after controlling for self-rating, grade remains a significant predictor of completion time. Alternatively, you could say that after controlling for class grade, self-rating is not a significant predictor of completion time. Which phrasing you choose should depend on which of the two predictor variables is of primary interest (from what you indicate, it sounds like self-rating is of primary interest, and hence the second sentence may be more appropriate).

As for "what's going on", it's hard to say with any certainty without access to the data and further analyses. It could be the case that grade is simply a much more important predictor of completion time than self-rating, and hence the effect of self-rating is masked by putting this other variable in the model. Also, if grade is strongly associated with self-rating, then the univariate model including self-rating as the only predictor may be measuring some combination of the effect of grade and self-rating; the bivariate model will "separate" these effects.

Hope this helps.

- J
.