©1999 Johanna Rothman

I was recently at a presentation at which a well-known measurement guru spoke. He said that most people don't know the value of a basic measurement: the cost to fix a defect. I was surprised, so at the next few conferences, I asked people in my presentations how much it cost their organizations to fix a defect.

Although I polled many people, only one person knew what it cost his company to fix a defect. If you're not sure either, here is an estimation technique to measure that cost:

In system test, when people are 100% dedicated to finding and fixing defects, count the number of fixes. You know how many people (developers, testers, and anyone else) worked on the project, and you know the duration of system test. The average cost to find and fix a defect is:

Average cost to fix a defect = (Number of people * number of days/Number of fixed defects) * cost per person-day

Note that the number of defects you find isn't enough information, it's the number of defects you fix. Detecting defects is only the first step. Locating the failure, deciding how to fix it, developer-testing the fix, system testing the fix, and looking for other defects this fix caused is why the fix value is what's important. In these three examples, I rounded slightly, because I care about the magnitude of the cost, not the dollars and cents of the cost.

One group of 7 people were able to fix only 143 out of 245 defects found in system test, over a period of 65 days. It took them 455 person-days (7 people * 65 days) to fix those 143 defects. That's about 3.2 days per fix. Assuming a person-day costs $500, their average cost to fix a defect was about $500 * 3.2 = $1600 per fix.

Another group of 5 people fixed 165 defects over a period of 35 days. It took them (35 days * 5 people) 175 person-days to fix 165 defects, about 1 defect per day. It cost them about $500 per fix.

A third group of 8 people fixed 50 defects over a period of 30 days. It took them (30 * 8) 240 person days to fix the 50 defects, about 4.8 days per fix. It cost them about $2400 per defect fix.

There is no standard cost to fix a defect. The cost to fix a defect changes for different projects and different organizations. If your costs are higher than you would like, use the data to help change how you develop products.

Tags: metrics, project management