This is one of the most effective measures. The number of defects found in a production is recorded. The only issue with this measure is it can have latent and masked defects which can give us the wrong value regarding software quality.

Defect seeding is a technique that was developed to estimate the number of defects resident in a piece of software. It's an offline technique and should not be used by everyone. The process is the following: we inject the application with defects and then see if the defect is found or not. So, for instance, if we have injected 100 defects we try to get three values. First how many seeded defects were discovered, how many were not discovered, and how many new defects (unseeded) are discovered. By using defect seeding we can predict the number of defects remaining in the system.

Test effectiveness is the measure of the bug-finding ability of our tests. In short, it measures how good the tests were. So effectiveness is the ratio of the measure of bugs found during testing to the total bugs found. Total bugs are the sum of new defects found by the user plus the bugs found in the test. The following figure explains the calculations in a pictorial format.