Defect Leakage

What is Defect Leakage?

Sometimes, one or more defects gets missed or unidentified, while testing the software application and these defects gets revealed and shows their presence post application release at the production site or at the end-user site. The defects which needs to be identified and caught during the system or the user acceptance testing at testing site, is reported at the production environment by the end users is called defect leakage. It is also known by the name of the bug leakage.

Besides, perceiving the defect leakage as a term to define the occurrence of the defects at the production site, it may also be seen as a metric to measure the efficiency of the testing process.

What could be the reasons for the defect leakage?

There could be any reason behind the defect leakage as any loopholes or gaps or deficiency or carelessness in the Quality assurance or in the Quality Control process may lead to the scenario of defect leakage. Below given are some of them.

  • Inadequate or lack of knowledge and understanding of the developed application and its domain may results into the production of inappropriate test scenarios and cases and subsequently lead to the situation of defect leakage.
  • Quality of the data imported from the production environment.
  • Difference in the test and the production environment.
  • Poorly designed test cases, not strictly adhering to requirements and norms.
  • Incorrect deployment of the application at test site of at production site.

How to Calculate Defect Leakage?

Since Defect leakage could be used as a metric to measure the efficiency of the testing, it may be measured and assessed for the whole software testing process or for each different levels of the testing in terms of measurable units, numbers, percentage, etc. This helps in reconsidering the testing process, approaches and techniques used as per the calculated defect leakage.

The mathematical formula to calculate the defect leakage could be seen below.

Defect Leakage(in %age) = {No. of defects found in UAT/Total No. of defects found during QA testing( including UAT)}*100
Where UAT is the User acceptance testing done at the production site by the end users.

For Example, 35 defects were found during UAT at the production site by the end users and 140 defects were found during the whole QA process including both the testing site and the production site. Therefore, the defect leakage would be

= (35/140)*100
= 25%
The defect leakage percentage is 25%.


It is almost impossible to identify and detect all the defects present in the software during the testing carried out at the testing site as some defects could only be explored when the software application is subjected to real or production environment. However, using the defect leakage metric, a testing team may try to increase the testing efficiency by revising the testing process and subsequently in bringing down the discovery of defects at the production site.