March 3, 2011, 9:40 p.m.
posted by concurre
Estimation and planning are related topics, but estimation is not planning, and planning is not estimation. Estimation should be treated as an unbiased, analytical process; planning should be treated as a biased, goal-seeking process. With estimation, it's hazardous to want the estimate to come out to any particular answer. The goal is accuracy; the goal is not to seek a particular result. But the goal of planning is to seek a particular result. We deliberately (and appropriately) bias our plans to achieve specific outcomes. We plan specific means to reach a specific end.
Estimates form the foundation for the plans, but the plans don't have to be the same as the estimates. If the estimates are dramatically different from the targets, the project plans will need to recognize that gap and account for a high level of risk. If the estimates are close to the targets, then the plans can assume less risk.
Both estimation and planning are important, but the fundamental differences between the two activities mean that combining the two tends to lead to poor estimates and poor plans. The presence of a strong planning target can lead to substitution of the target for an analytically derived estimate; project members might even refer to the target as an "estimate," giving it a halo of objectivity that it doesn't deserve.
Here are examples of planning considerations that depend in part on accurate estimates:
Creating a detailed schedule
Identifying a project's critical path
Creating a complete work breakdown structure
Prioritizing functionality for delivery
Breaking a project into iterations
Accurate estimates support better work in each of these areas (and Chapter 21, "Estimating Planning Parameters," goes into more detail on these topics).