I'm sorry if this question is obvious to some of you. It's just that I have a test that I would like to analyse using Receiver Operating Characteristic and I'm not sure how to proceed.

I have a test. Below a certain threshold (80%) the test results are basically random (true or false of the test are pretty meaningless). Above the threshold, the results become meaningful.

I thought that what I should do is delete the results below the threshold of 80% and to work with the meaningful results. If I delete below the threshold, I have 635 positive test results and 13 negative. This means that above the threshold of 80% where the test starts to become meaningful, the test is true 98% of the time.

However, I plotted the data above 80% in SPSS and I get a strange looking ROC curve. Here is what it looks like...

View attachment 3713

Now what I have done is used the use the test variable values above 80% (the test variable is a percentage) and ignored the values below 80% which are pretty random. I have used a state variable of 1 for true test result and 0 for false test result...

I was hoping that someone could throw some light on this problem. Why is the curve like it is and how do I use ROC with this particular test...????

Many thanks for any help...

Flurry