Tenet: Base your decisions on data and metrics, not intuition and opinion

If you have only walked in the dark, you will have never known the clarity that light brings. – The author

I was giving a presentation once to my CEO about the current state of play within our organisation. I was Development Manager at the time, so testing was not my primary focus. During the presentaiton, I couldn’t resist including a couple of testing related slides. The first slide showed an example defect trend graph, which I used to illustrate the sort of information that should be generated by the Test Manager to assist with the day to day decisions. The second slide was the same graph with the data removed so that only the two axes remained, illustrating the lack of information available when there aren’t any testers logging issues.

Steve McConnell used a brilliant analogy in Code Complete, where he compares testing to a bathroom scale when you are trying to loose weight. He highlights that the scale does not help you loose weight at all. The scale is merely an indicator of your progress towards your goal.

In my way of thinking, to extend Steve’s analogy, a test team is more like a weight loss clinic. The statistics and metrics that they produce are like the weekly weigh in, and blood test results that tell the real story of how you are progressing.

Government health warning: Metrics can be addictive

I don’t smoke, but testing metrics are like a cigarette habit, once you are used to having them, it is almost impossible to give them up. You may be able to go for short, painful, stints without them, but you know it is a case of when they will be back, not if.

Metrics can provide insights and answers to curly questions such as: When will the product ship? The simple answer is to average the number of bugs fixed per day, and divide the total number of bugs by the average. That is approximately how many days until you reach zero bugs. So, if you are fixing 5 bugs per day and you have 200 active bugs, the earliest that you will ship is in 40 working days time. If you want to ship sooner, you will need to stop adding features and focus on fixing more bugs. The same information can be used in reverse to calculate a maximum allowable bug count. Say you only have 40 days until your desired ship date, and you are fixing 5 bugs per day as in the previous example. If you active bug count is over 200 today, you will probably miss your target. This number continuously decreases so in 2 weeks time, with 30 working days to go, your bug count should be at the 150 mark if you are going to hit your ship date.

Interpreting the results sometimes makes you feel more like a statistician than a test manager. But trust me, it is well worth the effort.


Steve McConnell 1993, Code Complete, Microsoft Press.