Although you can type estimates directly into task fields in Project, an Excel spreadsheet has several advantages. First of all, many of the people who estimate tasks are familiar with Excel but not Project. (In addition, estimators may not have a copy of Project, but they almost certainly have Excel.) Excel makes it easier to adjust estimates than Project, for instance, recalculating hour estimates into days or choosing worst-case estimates for high-risk tasks.
You can export work package tasks in Project to build a simple estimating spreadsheet in Excel. You can divvy up the tasks for each estimator into a separate Excel workbook, and then reassemble the results in one master spreadsheet that you import back into Project. If you ask for three estimates (Getting Estimates from the Right People), you can use Excel functions to set your estimated values.
To get estimates into a Project file, you first export the task names and WBS codes for work package tasks to an Excel workbook. Because WBS codes are unique, you can use them as a link between tasks in the spreadsheet with the corresponding tasks in Project. Then, after you complete your spreadsheet of estimate numbers, you import those values into the existing tasks in your Project file.
If you're estimating from the bottom up, the only tasks you estimate are the work packages—the lowest level tasks in the Project outline. You don't have to export summary tasks because ...