Predicting How Much Your Schedule Will Slip
December 5, 2007 – 12:20 amI have written here before on how much detail you should put in your project plan. In summary, my proposal was to go with sufficiently small detail to be able to keep up with things on a daily basis. A few days ago, Johanna Rothman over on the Managing Product Development blog put forth an interesting theory that is another reason to keep your project plan detailed: Estimation Units are a prediction of schedule slippage. One of the participants of her workshop suggested this simple theory: If you estimate in days, you’ll be off by days. If you estimate in weeks, you’ll be off by weeks.”
This is such an obvious-sounding theory that it requires a bit of thought to accept. After all, if it were that simple to keep your project on track, we all would have found it by now, right? I went back and did some research in a random sample of old projects though, and it holds true- when people gave me estimates such as “four weeks” and the date slipped, it came in 1-2 weeks late. When they gave me estimates in days, it slipped by days, not weeks. And sure enough, in the few cases where there was a mega-task estimated in months, when there was slippage, it wasn’t in days, or weeks, or hours… it was by a month or more. Rohanna goes on to include in her article her theories as to why this happens, and I have to agree- the bigger the estimation unit used, the less thought that the estimator has given the task. Someone who says “20 days” has thought more about a task than someone who says “four weeks”, and certainly more than someone who says “one month”.
Consider this when you are doing estimates in your project plans. More work yes, but when deadlines matter, you can’t afford not to plan enough.
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