Defect Detection Percentage (DDP)

ISTQB Glossary  definition 

The number of defects found by a test phase, divide data by the number found by that test phase and any other means afterwards

In Simple English,

So, out of all the defects found how much of it were found in the test phase for which defect detection percentage is needed.

Field Notes 

1. The measurement is a percentage, the names so. This means that the value should never be above 100. Though this is a basic fact, at times people miss this.
2. This is a relative measure of how much, not how many defects.
3. When you publish this metric, ensure that you mention any phase that was omitted. Some times the project defines unit test as out of scope as a external vendor might be doing the development, in such cases the unit test defect counts might not be tracked as part of the project and will be not counted for this metric too.
4. This metric says a lot about the productivity of the team that was involved in each test phase.
5. Caution: This metric should not be used as the only measure to judge the team involved. sometimes a difference in understanding of domain, training issues, schedule and quality of code available for testing and several other factors determine the defect detection percentage.
6. One common use of this metric is to check if each phase found fewer defects than the previous. meaning the code kept getting better and better. 
7. One short fall of this metric is that it can be used reliably after all testing is done for an analysis of what went wrong.

For Example:

Say the project has 4 distinct test phases. 

1. Unit Testing

2. System Testing

3. System Integration testing 

4. User acceptance testing

To keep things easy to understand lets say totally 100 defects were found in the project.

lets say following is the break down of number of defects found in each phase.

1. Unit Testing - 10 defects

2. System Testing -20 defects 

3. System Integration testing  - 30 defects

4. User acceptance testing - 40 defects

Now its quite easy to say 10 of the 100 defects were found in Unit testing, making the defect detection percentage for Unit test phase as 10%

just like this, each test phase gets a percentage value showing how many of the total defects were found in it.

1. Unit Testing - 10 %

2. System Testing -20 % 

3. System Integration testing  - 30 %

4. User acceptance testing - 40 %

so the formula is  

Defect detection percentage in phase A = ( defects found in phase / total defects found in all phases together)  x 100