DRE is also useful to measure effectiveness of a particular test like acceptance, unit or system testing. Below figure shows defect numbers at various software cycle level. The + indicates that defects are input to the phase and – indicates that these many defects where removed from that particular phase. For instance in requirement phase 100 defects where present, but 20 defects are removed from the requirement phase due to code review. So if 20 defects are removed then 80 defects get carried to the new phase (design) and so on.
Figure: - Defect injected and removed per phase
First let’s calculate simple DRE of the above diagram. DRE will be total bugs found in testing divided by total bugs found in testing plus total bugs found by user that is during acceptance testing. So below diagram gives the DRE for the above values.
Figure: - DRE calculation
Now lets calculate system DRE of the above given project. In order to calculate the system DRE we need to take number of defects found during system divided by defects found during system testing plus defects found during acceptance testing. Below figure shows the System DRE calculation in a step by step fashion.
Figure: - System test DRE calculation
Unit testing DRE calculation will be similar to system testing DRE. As you can see from the below figure its nothing but Number of defects found during Unit testing divided by Number of defects found during unit testing plus Number of defects found during system testing.
Figure: - Unit test DRE calculation
One of the important factors to be noted while calculating unit test DRE is that we need to exclude those defects which can not produced due to limitation of unit testing. For instance passing of data between components to other. In unit testing because we test it as a single unit we can never reproduce defects which involves interaction. So such kind of defects which can not be produced due to the nature of the testing itself should be removed to get accurate test results.