Info

0.67

1.00

1.67

0.83

0.45

10.05

2.79

0.69

14.98

Your next logical conclusion might be to add the Standard Deviation column to get the standard deviation for the project. Unfortunately, you cannot add the standard deviations because you will come out with a number that is much too high. Totaling the standard deviations assumes that all the tasks will run over schedule, and that's not likely. It is likely that a few tasks will run over but not every one of them. So now you're probably wondering how to calculate the magic number.

You might have noticed an extra column at the right called SD Squared. This is the standard deviation squared—or for those of you with math phobias out there, the standard deviation multiplied by itself.

Once you have calculated the standard deviation squared for each activity, add the squares, for a total of 14.98. There's one more step, and you're done. Take the square root of 14.98 (you'll need a calculator) to come up with 3.87. This is the standard deviation you will use to determine your range of projected completion dates. Here's a recap of these last few calculations:

Total expected value = 103.00

Sum of SD Squared = 14.98

Square root of SD Squared = 3.87

You can now make the following predictions regarding your project:

■ There is a 68.26 percent chance that the project will be completed in 99.13 days to 106.87 days.

■ There is a 95.44 percent chance that the project will be completed in 95.26 days to 110.74 days. 