Weibull Regression for risk curve

praco

Hi evryone!

I need your help.

I have a set of data with 2 variables: one is binary (f: y/n) the other one continu (a).

The higher "a", the more chance to have f=yes.

I need to use the Weibull regression to find the 0.5 probability to get f=yes. This is like a logistic regression, but one of the advantages of using the weibull is that the risk curve goes through 0 and 1, unlike the logistic regression.

I found a Weibull tool in the fit curve menu, but the curve stops at .8...

Can someone help me?

 

thanks 

Courbe d'ajustement.png

Mark_Bailey

I don't know what you mean by "the (Weibull) risk curve goes through 0 and 1, unlike the logistic regression." Do you mean that you are using a Weibull CDF to model the response? Are you referring to the values (0,1) that you use to code the binary levels or the probability of fracture?

 

The regression will not necessarily achieve 0 or 1 if the range of the predictor is limited such that the asymptotes are not achieved. I tried to read the Ax Dt values from the plot and then performed a nominal logistic regression.

 

 

You do not need to achieve the limiting 0 and 1 asymptotes over the observed range to apply inverse prediction to estimate Ax Dt for probability = 0.5.

 

The shape of the curve is constrained by the function you use for your model. The binary nature forces a transition from one asymptote to the other. The higher asymptote is limited to 0.8 because 4 out of 5 responses are 1 and 1 out of 5 is 0. This observation will also limit the logistic curve.