I am requesting help in understanding ROC curves as generated by PROC LOGISTIC. I am using the data in Kleinbaum DG and Klein M : Logistic Regression. THE data set is Knee fracture or kneefr. The outcome variable is fracture   and is binary. The data set is located  at http://web1.sph.emory.edu/dkleinb/logreg3.htm#data As kneefr.sas7bdat or kneefr,dta   The predictor variables are: FLEX  [Flex the knee]  Binary  yes, no WEIGHT ability to put weight on knee [yes no] AGECAT  patients age greater or less then 55years HEAD    Injury to knee head [yes no] PATELLAR injury to patellar [yes no] >>>>>>>>>>  THE CODE COPIED FROM KLEINBAUM and KLEIN proc logistic data = kneefr descending;                 model fracture = Flex weight agecat head patellar /pprob = .00 to .50 by .05 ctable;                 run;          proc logistic data = kneefr descending;                 model fracture = Flex weight agecat head patellar /outroc =cat;                 run;          Proc Print Data=CAT (obs=10); run;          SYMBOL = PLUS interpol = RC;    proc gplot data=CAT;                 plot _SENSIT_*_1MSPEC_ ;                          run; >>>>>>>>>>>>>>>>>>>>>>>>>>>          MY QUESTIONS I can understand that from a logistic regression and feeding in the data for an individual patient one obtains a ln(0dd) for that patient.  From this the probability of fracture for the individual.   I do not understand how SAS calculates predicted probabilities by .00 to .50 by 0.5. Nor do I understand what they mean in relation to the number of fractures and /or no fractures at a probability level  when printed as the ‘ctable’ or [cut off table].   The print out of the Cat data gives the probability for I assume each patient. Obs 10 for instance is given as 0.22898.  Does this mean given the cut off set at 0.300  this patient is in the positive class either as a TP or a FP.   I am using SAS university Edition and the code for Gplot does not run.   I note that with PROC PLOT a curve is produced. Is this because the University Edition does not support GPLOT?   Is this a correct assumption?   5 Is it possible to obtain a histogram of the _pos_ and _neg_ cases with the probability cut off values on the x axis.     I thank you in advance for any assistance.     ... View more