Estimates: Precision vs. Accuracy

©2003 Johanna Rothman www.jrothman.com

Jim, a new project manager, struggled to define the project?s parameters: schedule estimate, people estimate, requirements outline, and necessary capital equipment. Jim proudly walked into his manager?s office, and proceeded to walk through his project plan discussing these numbers:

64.1 calendar weeks

12.125 people

ROI (return on investment) of 7.5 months

Jim?s manager, Dave, stopped him, smiling, and said, ?So, Jim, where?s this .125 person coming from? And what?s .1 of a week?? Jim was a little stumped, and said, ?Oh, I guess we?ll borrow someone else. I thought you?d want to see a project plan with the fewest number of people. One-tenth of a week is part of a day.? Jim?s manager said, ?Well, we only have whole people, so I need a plan with either 12 or 13 or whatever you think the right number of people is for this project. And, while you?re at it, although I would love to plan a release for a Monday at 9:37am, we both know that?s not going to happen. Please work up an estimate that?s more accurate, but not as precise. And, while you?re at it, show me our options?how many people for how many weeks, giving me ideas for how we would staff a shorter project with more people and a longer project with fewer people.?

Precision is the exactness of the measurement, the number of decimal places. Accuracy is how close you are to the estimate. When I worked with robots, precision meant how close to a specific place in space we could move the robot arm once. Accuracy meant how many times we could make the robot arm move to that precise place. Jim?s estimate was certainly precise, but estimates are more useful when they are accurate, close to the mark. Projects are composed of numerous predicted estimates, so we can?t expect accurate estimates, except to an order of magnitude. Over time, you can learn how to estimate so that your estimates (predictions) are close to your actual schedule.

When you talk about estimates, you?re in much better shape if you choose accuracy (how close you are to the prediction) over precision (exactness). As you move closer to accomplishing milestones, you can re-estimate with greater precision.

One way to discuss a schedule estimate is to deal with a range. Jim could have said, ?I?ll need 13 people for about 64 weeks. Or, I can do this project with more difficulty if I have 20 people for about 50 weeks. Or, if we take out this specific performance requirement, we?ll need 18 people for about 48 weeks.?

Each of the ?abouts? in the previous sentence exists for accuracy. Schedule estimates are just that, an estimate. In my experience, putting an ?about? in front of a schedule estimate is more helpful than a range (?62-65 weeks?). I?ve met too many senior managers who don?t hear the second number, just the first. When I use an about, I can choose a number that makes sense to me to use to senior management.

However, some of you work for managers who prefer a range of estimates, with a confidence for each point in the range. This is particularly helpful if you use a graph to explain your confidence. (see Figure 1.)

Figure 1: Estimates: Precision vs. Accuracy

Figure 1: Estimates: Precision vs. Accuracy

The graph is a vehicle for starting the discussion about why you have how much confidence when. You can explain why you only have 50% confidence that you will meet the 3/1 requested release date, and you can explain why your confidence improves over time. With this graph, you?ll especially want to explain the difference in confidence between 6/1 and 7/1. The project manager who generated this graph was concerned about the amount of rework (problem finding and fixing) at the end of the project, and wasn?t absolutely sure the project team could finish the rework for 6/1. When he explained how the last three releases had used more than the planned four weeks, his management understood why his confidence level was less than 100% for the 6/1 date.

If you have a sponsor who says, ?Just give me a number, and give it to me now,? you have numerous choices about the date you provide. You can choose a date somewhere in your range, depending on how risk averse you are. Frequently, sponsors who need dates without a discussion of estimates really do want just a date you can promise, not the first possible date the project could be ready if Murphy?s Law didn?t come visit your project and all the planets were aligned. If your sponsor is not willing to use the date you provide, you?re dealing with the ?Bring me a Rock? game. If you have a sponsor who appears to not be willing to consider why you don?t have a date, you have several options: make an appointment to talk about project schedule estimates; discuss your sponsor?s concerns and what matters to your sponsor; or find a new sponsor or new organization.

Choose some technique, ?about? an amount of time, dates with confidence attached, or a date in the middle of your range, to help your project sponsor see that the schedule or the budget or the ROI, or whatever number you?re discussing is an estimate.

Be precise at the end of a project, when you file your project information, and manage the data that was part of the project?s outcome. In the meantime, be as accurate as you can, by using approximations, ranges, and ?about?. Jim now works for accuracy at the beginning of his projects, and precision at the end.



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