model-comparison

I'm doing some exploratory work where I use dredge{MuMIn}. In this procedure there are two variables that I want to set to be allowed together ONLY when the interaction between them is present, i.e. they can not be present together only as main effects. Using sample data: I want to dredge the model fm1 (disregarding that it probably doesn't make sense). If the variables GNP and Population appear together, they must also include the interaction between them. require(stats); require(graphics) ## give the data set in the form it is used in S-PLUS: longley.x <- data.matrix(longley[, 1:6]) longley

I am aware of the concept of Precision as well as the concept of Recall. But I am finding it very hard to understand the idea of a 'threshold' which makes any P-R curve possible. Imagine I have a model to build that predicts the re-occurrence (yes or no) of cancer in patients using some decent classification algorithm on relevant features. I split my data for training and testing. Lets say I trained the model using the train data and got my Precision and Recall metrics using the test data. But HOW can I draw a P-R curve now? On what basis? I just have two values, one precision and one recall.