Maintainability Index Range and Meaning
Another question:
The maintainability index has been re-set to lie between 0 and 100. How and why was this done?
The metric originally was calculated as follows (based on the work in Carnegie Mellon University although we modified the Halstead Volume calculation a little since we don't include comments anywhere in our calculation): Maintainability Index = 171 - 5.2 * ln(Halstead Volume) - 0.23 * (Cyclomatic Complexity) - 16.2 * ln(Lines of Code)
This meant that it ranged from 171 to an unbounded negative number. We noticed that as code tended toward 0 it was clearly hard to maintain code and the difference between code at 0 and some negative value was not useful. I'll post some tech ed sample code showing very low maintainability or you can try on your own code to verify. As a result of the decreasing usefulness of the negative numbers and a desire to keep the metric as clear as possible we decided to treat all 0 or less indexes as 0 and then re-base the 171 or less range to be from 0 to 100. Thus, the formula we use is:
Maintainability Index = MAX(0,(171 - 5.2 * ln(Halstead Volume) - 0.23 * (Cyclomatic Complexity) - 16.2 * ln(Lines of Code))*100 / 171)
On top of that we decided to be conservative with the thresholds. The desire was that if the index showed red then we would be saying with a high degree of confidence that there was an issue with the code. This gave us the following thresholds (as mentioned in this blog previously):
For the thresholds we decided to break down this 0-100 range 80-20 so that we kept the noise level low and only flagged code that was really suspicious. We have:
- 0-9 = Red
- 10-19 = Yellow
- 20-100 = Green
