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Both quantity and type of errors are affected by project size. You might not think that error type would be affected, but as project size increases, a larger percentage of errors can usually be attributed to mistakes in requirements and design, as shown in Figure 27-2. Figure 27-2. As project size increases, errors usually come more from requirements and design. Sometimes they still come primarily from construction (Boehm 1981, Grady 1987, Jones 1998)Cross-Reference For more details on errors, see Section 22.4, "Typical Errors."
On larger projects, construction errors can taper off to about 50 percent of the total errors; requirements and architecture errors make up the difference. Presumably this is related to the fact that more requirements development and architectural design are required on large projects, so the opportunity for errors arising out of those activities is proportionally larger. In some very large projects, however, the proportion of construction errors remains high; sometimes even with 500,000 lines of code, up to 75 percent of the errors can be attributed to construction (Grady 1987).
The data in this table was derived from specific projects, and the numbers might bear little resemblance to those for the projects you've worked on. As a snapshot of the industry, however, the data is illuminating. It indicates that the number of errors increases dramatically as project size increases, with very large projects having up to four times as many errors per thousand lines of code as small projects. A large project will need to work harder than a small project to achieve the same error rate. Cross-Reference The data in this table represents average performance. A handful of organizations have reported better error rates than the minimums shown here. For examples, see "How Many Errors Should You Expect to Find?" in Section 22.4. |
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