May 8, 2011, 5:49 p.m.
posted by concurre
Suppose you're preparing for a trip and deciding which suitcase to take. You have a small suitcase that you like because it's easy to carry and will fit into an airplane's overhead storage bin. You also have a large suitcase, which you don't like because you'll have to check it in and then wait for it at baggage claim, lengthening your trip. You lay your clothes beside the small suitcase, and it appears that they will almost fit. What do you do? You might try packing them very carefully, not wasting any space, and hoping they all fit. If that approach doesn't work, you might try stuffing them into the suitcase with brute force, sitting on the top and trying to squeeze the latches closed. If that still doesn't work, you're faced with a choice: leave a few clothes at home or take the larger suitcase.
Software projects face a similar dilemma. Project planners often find a gap between a project's business targets and its estimated schedule and cost. If the gap is small, the planner might be able to control the project to a successful conclusion by preparing extra carefully or by squeezing the project's schedule, budget, or feature set. If the gap is large, the project's targets must be reconsidered.
The primary purpose of software estimation is not to predict a project's outcome; it is to determine whether a project's targets are realistic enough to allow the project to be controlled to meet them. Will the clothes you want to take on your trip fit into the small suitcase or will you be forced to take the large suitcase? Can you take the small suitcase if you make minor adjustments? Executives want the same kinds of answers. They often don't want an accurate estimate that tells them that the desired clothes won't fit into the suitcase; they want a plan for making as many of the clothes fit as possible.
Problems arise when the gap between the business targets and the schedule and effort needed to achieve those targets becomes too large. I have found that if the initial target and initial estimate are within about 20% of each other, the project manager will have enough maneuvering room to control the feature set, schedule, team size, and other parameters to meet the project's business goals; other experts concur (Boehm 1981, Stutzke 2005). If the gap between the target and what is actually needed is too large, the manager will not be able to control the project to a successful conclusion by making minor adjustments to project parameters. No amount of careful packing or sitting on the suitcase will squeeze all your clothes into the smaller suitcase, and you'll have to take the larger one, even if it isn't your first choice, or you'll have to leave some clothes behind. The project targets will need to be brought into better alignment with reality before the manager can control the project to meet its targets.
Estimates don't need to be perfectly accurate as much as they need to be useful. When we have the combination of accurate estimates, good target setting, and good planning and control, we can end up with project results that are close to the "estimates." (As you've guessed, the word "estimate" is in quotation marks because the project that was estimated is not the same project that was ultimately delivered.)
These dynamics of changing project assumptions are a major reason that this book focuses more on the art of estimation than on the science. Accuracy of ±5% won't do you much good if the project's underlying assumptions change by 100%.