Sanity check

Once you add up all of your bottom-up estimates, you should arrive at a total close to the project manager's top-down estimate. If the estimates are not fairly close, your project contains some uncertainty that you need to examine.

The project manager might find that the bottom-up estimate is higher because it includes the tasks he or she did not consider in the top-down estimate. That's fine. However, if you have a major discrepancy and cannot find the reason, you need to revisit your estimates. As described earlier, you get better estimates if you let more people do the estimation and then calculate the averages. Do this for both your top-down and bottom-up estimates until the total bottom-up estimate is approximately the same as the top-down estimate.