To provide a reality check on these estimates, you might compare the four different approaches to effort estimates in this chapter:
Doing informal comparisons to past projects
Using estimation software
Using industry-average graphs
Using the ISBSG method
If you graphed the estimate ranges from these techniques, they would look like those shown in Figure 19-10.
Figure 19-10: Ranges of estimates derived by using the methods discussed in this chapter. The relative dot sizes and line thicknesses represent the weight I would give each of the estimation techniques in this case.
The graph shows a range of estimates from about 40 to 110 staff months. The ISBSG method and the industry-average graphs are both using industry-average data, so I would weight them less heavily than the methods that are based on historical data. With the informal comparison to past projects, I would weight the projects that are most similar (including closest in size) the most heavily.
All things considered, in this case I would present a range of 65 to 100 staff months with an expected outcome of 80 staff months. You might think the expected outcome should fall in the middle of the range of 65 to 100, but the effort will often fall toward the low end of the range because of the issues discussed in Section 1.4, "Estimates as Probability Statements."
Tip #88 | Not all estimation methods are equal. When looking for convergence or spread among estimates, give more weight to the techniques that tend to produce the most accurate results. |