More Confidence

Can I do better? Suppose I wanted to go out to the 2 sigma point. This would then lead to a probability of success of around 84 percent:

50% + ¹/ 2(68%) = 50% + 34% = 84%

This would bring up our odds to five-to-one, which any project manager would gladly accept. In fact, this would be standing Standish on its head: five successful projects for every unsuccessful one.

What would it take to get us there?

Well, I can do the math both ways, either starting from our original Plan A or from the 50/50 Plan B. For consistency's sake, let's begin with Plan A. The math is pretty much the same. I now have to go from 0.66 sigma to 2 sigma, increasing our altitude by a factor of 3. That means I must multiply the area of the base by a third, which in turn means that I must multiply each side by the square root of 0.333. And in our previous list of things we'd need to change simultaneously to achieve better results, we'd have to replace 18 percent with 42 percent.

Let's now summarize, using rough numbers so that we don't assign spurious precision to the model.^[9] Plan A has a probability of success of only around 20 percent. As we have seen, if we simultaneously reduce the difficulty of all four of the base parameters (scope, quality, speed, and frugality) by about 20 percent, we get Plan B, which has a 50 percent probability of success. To achieve an 85 percent success rate, we'd need to reduce the difficulty on the base parameters by around 40 percent relative to Plan A. Table 9.1 summarizes these relationships.

^[9] Remember also the detail we ignored earlier about the mean and median not being identical for the lognormal distribution. Here is where we can bury some of that approximation.

Table 9.1. Results of Using the Pyramid Model and Lognormal Distribution

Plan	Description	Location on Lognormal Curve	Probability of Success	Values for Base Parameters
A	Navïe and most likely starting point	0.67 sigma	20 percent	Per Plan
B	More realistic	1 sigma	50 percent	Reduced by 20 percent relative to A
C	High efficiency	2 sigma	85 percent	Reduced by 40 percent relative to A

Clearly we've gone way out on a thin limb here, but the numbers in Table 9.1 represent the pyramid model's predictions, based on the lognormal distribution for project outcomes and a constant volume assumption.