Logistic regression

[Dinan]

Hello everyone...

I'm trying to create a graph similar to the FIG.
I did the analysis in proc logistic and I found a quadratic effect and I created the graphic,

But I don't know how to calculate the maximum predicted value and the confidence interval.

Thanks so much in advance for your help...

 

[trinker]

What program are you using?

 

[Dason]

"proc logistic" implies they're using SAS.

 

[trinker]

missed that Dason. This question seems to be more about SAS and not logistic regression. Perhaps it should be moved there rather than bio stats.

Dinan, I am not a SAS user but some others here are. Hopefully they see this post and provide assistance.

 

[Dason]

It could be about how to do code it - but the theory behind how you backtransform and get a confidence interval around what predictor value gives you the maximum is interesting and non-trivial as well.

 

[Dinan]

Thanks for your help Trinker,
I forget to say, I used SAS...

Below the paragraph where the authors describe how they did this analysis
"For each group of cows, the first derivative of pregnancy rate with respect to follicle size was solved for its root, in the range of observed follicle sizes, to obtain the follicle size resulting in maximum predicted pregnancy rate. A 90% confidence interval for the predicted maximum was calculated by back-transforming the corresponding critical values for their linear predictors and was used to establish the lower critical values where pregnancy rate was significantly reduced (P  0.05) relative to the maximum."