20.7 Schedule Estimation and Staffing Constraints

Nominally, you can compute the average team size by taking the effort estimate and dividing it by the schedule. If you've estimated a 12-month schedule for a project of 80 staff months, your average team size is just the staff months divided by the schedule, 80 divided by 12, which is 6 to 7 team members.

The schedule estimates produced in this chapter produce the nominal schedule for a project at a particular level of effort. These techniques assume that, whatever the nominal schedule is, you'll be able to adjust your team size to fit the level indicated. If you're in a position to apply an average of 6 to 7 people to the project estimated in this chapter, you should be able to achieve the combination of 80-staff-month effort and 12-calendar-month schedule that's been estimated.

What if you have only 4 people to work on the project? What if you're already at a point in the project where you're assigning specific tasks? What if you have 10 people who are each available two-thirds of the time? What if the team is already intact and doesn't need any ramp-up time? The formulas in this chapter don't account for such factors: these formulas are macro estimation techniques that are appropriate only in the early stages of medium-to-large projects.

Medium and large projects typically experience some ramp up of team members from the beginning to the middle of the project, and some ramp down in the final stages. A medium-sized project might average 15 people, but it might start with 5, peak at 20, and end with 10.

Smaller projects more often use "flat staffing"—the whole team starts on Day 1 and continues through the end of the project. If your schedule estimate is 12 months but your plans show that, based on people's availability, you will need 15 months to actually apply 80 staff months of effort, your plans should take precedence over the original schedule estimate.

The purpose of the schedule estimates in this chapter is not to predict your final schedule to the day but to provide a sanity check on your schedule-related plans. Once you've used schedule estimation to ensure that your plans are reasonable, detailed planning considerations (such as who is available when) will take precedence over the initial schedule estimation described.

Tip #96

Use schedule estimation to ensure your plans are plausible. Use detailed planning to produce the final schedule.