114 n Interpreting the CMMI
At Level 2, measures are collected, stored in a database per project, bubble up
to an organizational database in Level 3, are reviewed for consistency and accuracy
at Level 3, and then, at Level 4, have statistically-based controls applied to them.
What to put under statistical control depends on where the problems are in your
organization, and which processes and measures will add value to your management
techniques. This statement implies that not all processes must be put under
statistical control. But, we suggest that for Level 4, and for this process area in
particular, the OSSP must be understood from a statistical point of view.
The most common measurements we see in use for this process area are size,
effort, cost, schedule, and product defect density. The measurements for these data
points are usually displayed in ranges and not by absolute points. Subsets of measures
can be generated to be applied based on domains, new development versus
maintenance, and type of customer. The CMMI has added some examples of candidate
subprocesses to select for performance measurement and modeling.
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