Although seemingly straightforward, comparing the defect rates of software products involves many issues. In this section we try to articulate the major points.
To define a rate, we first have to operationalize the numerator and the denominator, and specify the time frame. As discussed in Chapter 3, the general concept of defect rate is the number of defects over the opportunities for error (OFE) during a specific time frame.
We have just discussed the definitions of software defect and failure. Because failures are defects materialized, we can use the number of unique causes of observed failures to approximate the number of defects in the software. The denominator is the size of the software, usually expressed in thousand lines of code (KLOC) or in the number of function points.
In terms of time frames, various operational definitions are used for the life of product (LOP), ranging from one year to many years after the software product’s release to the general market. In our experience with operating systems, usually more than 95% of the defects are found within four years of the software’s release. For application software, most defects are normally found within two years of its release.