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Does Defect density measure QA or DEV effectiveness? [Resolved]

Most Defect density metrics are like this:

Number of bugs / Size (KLOC, Story Points, Function points..)

I have read that it measures the effectiveness of QA but I do not get it:

  • I can have a super senior developer and a sloppy QA. Therefore, the sloppy QA guy finds nothing because there is nothing.
  • I can have a bad developer and a good QA guy, who finds most of the issues.

Therefore, how this could be measuring the QA? In my view, it reflects both quality of dev and effectiveness of QA.


Question Credit: John V
Question Reference
Asked October 10, 2018
Posted Under: Programming
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3 Answers

Defect density measures the number of defects found per unit of code measurement (never ever use lines of code for this), for a given period of time. The higher the number, the greater the density of bugs.

The density of bugs can be taken to be an inverse measure of the quality of the app. The lower the density (ie, the less defects reported), the higher the quality of the code.

But being a simple metric it has flaws:

  • If I simply do not test the code, my defect density is zero: perfect quality! Of course in reality, most untested code is far from good quality.
  • Conversely, for the same piece of code, I would get way more defects reported by super-zealous testers than I would for a lacklustre dev asked to do some testing. So I now have two densities (and thus two quality measures) for the one piece of code. And since I'm judging those testers on the number of defects raised, I'm incentivising them to score the quality as low as possible.
  • And of course, there's the issue of early testing. Pair a developer and tester during a sprint, and the output will likely be low in bugs. But it's extremely unlikely that the defects they found during the sprint will be recorded, so I've no measure of quality unless I retest just to gain my metric.

Defect density is just a metric. To be able to read more into it (quality of code, effectiveness of testing, likelihood of the app containing significant bugs etc) requires a heavy dose of subjectivity. Unless you know how effective your testing is, defect density won't be a reliable quality measure for example. But if you can't use metrics to measure effectiveness of testing, how do you measure it? So a subjective measure is needed.


credit: David Arno
Answered October 10, 2018

It was probably refering to defects found after release. ie by users not QA.

In this case, it is a measure of QA effectiveness as poorly coded work would be easier to find bugs in and hence not release. Very well, but not perfect code, might have a couple of minor, hard to find bugs slip through, but not many.

Of course it could also measure project manager-careing-more-about-deadlines-than-bugs-ness


credit: Ewan
Answered October 10, 2018
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