1.5 Common Definitions of a


1.5 Common Definitions of a "Good" Estimate

The answer to the question of what an "estimate" is still leaves us with the question of what a good estimate is. Estimation experts have proposed various definitions of a good estimate. Capers Jones has stated that accuracy with ±10% is possible, but only on well-controlled projects (Jones 1998). Chaotic projects have too much variability to achieve that level of accuracy.

In 1986, Professors S.D. Conte, H.E. Dunsmore, and V.Y. Shen proposed that a good estimation approach should provide estimates that are within 25% of the actual results 75% of the time (Conte, Dunsmore, and Shen 1986). This evaluation standard is the most common standard used to evaluate estimation accuracy (Stutzke 2005).

Numerous companies have reported estimation results that are close to the accuracy Conte, Dunsmore, and Shen and Jones have suggested. Figure 1-6 shows actual results compared to estimates from a set of U.S. Air Force projects.

image from book
Figure 1-6: Improvement in estimation of a set of U.S. Air Force projects. The predictability of the projects improved dramatically as the organizations moved toward higher CMM levels. [1]

Figure 1-7 shows results of a similar improvement program at the Boeing Company.

image from book
Figure 1-7: Improvement in estimation at the Boeing Company. As with the U.S. Air Force projects, the predictability of the projects improved dramatically at higher CMM levels.

A final, similar example, shown in Figure 1-8, comes from improved estimation results at Schlumberger.

image from book
Figure 1-8: Schlumberger improved its estimation accuracy from an average overrun of 35 weeks to an average underrun of 1 week.

One of my client companies delivers 97% of its projects on time and within budget. Telcordia reported that it delivers 98% of its projects on time and within budget (Pitterman 2000). Numerous other companies have published similar results (Putnam and Myers 2003). Organizations are creating good estimates by both Jones's definition and Conte, Dunsmore, and Shen's definition. However, an important concept is missing from both of these definitions—namely, that accurate estimation results cannot be accomplished through estimation practices alone. They must also be supported by effective project control.

[1]The CMM (Capability Maturity Model) is a system defined by the Software Engineering Institute to assess the effectiveness of software organizations.




Software Estimation. Demystifying the Black Art
Software Estimation: Demystifying the Black Art (Best Practices (Microsoft))
ISBN: 0735605351
EAN: 2147483647
Year: 2004
Pages: 212

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