These graphs represent the measured accuracy rates of SpamAssassin 3.2.0 in our tests, at various settings of 'required_score', with various types of rules in use.
The orange line represents false positives (ham mails marked as spam), and the blue represents false negatives (spam mails marked as ham); the lower they are, the more accurate the filter is. Generally we want to pick a threshold value where the orange line is much lower than the blue line, since for most people, a false positive is a much more serious error than a false negative.
Mouse over each graph to explore values, or drag over a part of the graph to zoom in. (Thanks to amCharts for the great chart widget.)