Measurement of Maintainability Index (MI) with Testwell CMT++ and CMTJava (Complexity Measures Tools)

Testwell CMT++ and Testwell CMTJava support Lines-of-code-metrics, Halsteads Metrics, McCabe Metrics and the Maintainability Index (MI).

MI is a single-number value for estimating the relative maintainability of the code.

Maintainability Index is calculated with certain formulae from lines-of-code measures, McCabe measure and Halstead measures.

The measurement and track maintainability are intended to help reduce or reverse a system's tendency toward "code entropy" or degraded integrity, and to indicate when it becomes cheaper and/or less risky to rewrite the code instead to change it.

There are two variants of Maintainability Index: one that contains comments (MI) and one that does not contain comments (MIwoc). CMT++ and CMTJava calculate them both.
The tools calculate Maintainability Index for functions (and class/struct definitions), for files and for the whole software.

Meanings of the Maintainability Index (MI, with comments) values:

To realize the full benefit of MI, the maintenance environment must allow the rewriting of a module when it becomes measurably unmaintainable. The point of measuring the MI is to identify risk; when unacceptably risky code is identified, it should be rewritten.

Calculation of Maintainability Index

Maintainability Index is calculated on each function/class/struct, on each file and on all files together level.

Actually there are three measures:

• MIwoc: Maintainability Index without comments
• MIcw: Maintainability Index comment weight
• MI: Maintainability Index = MIwoc + MIcw

The general formulae for MI is the following:

        MIwoc = 171 - 5.2 * ln(aveV) -0.23 * aveG -16.2 * ln(aveLOC)

        MIcw = 50 * sin(sqrt(2.4 * perCM))

        MI = MIwoc + MIcw

Where

        aveV = average Halstead Volume (CMT++/CMTJava´s V) per module
        aveG = average extended cyclomatic complexity (CMT++/CMTJava´s v(G) ) per module
        aveLOC = average count of lines (CMT++/CMTJava´s LOCphy) per module
        perCM = average percent of lines of comments per Module

"module" is (in CMT++ case) a C-like function definition, a C++-like member function definition, or a class/struct definition.

further information about Maintainability Index:
Kurt D. Welker, Idaho National Engineering and Environmantal Laboratory: The Software Maintainability Index Revisited (August 2001)
 

further information about Testwell Complexity Measures Tools:
Testwell CMT++/CMTJava

last updated: 24.05.2011

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