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How do we estimate?

This post is from George Dinwiddie's blog by George Dinwiddie. Click here to see the original post in full.

There have been some web posts and twitter comments lately that suggest some people have a very narrow view of what techniques constitute an estimate. I take a larger view, that any projection of human work into the future is necessarily an approximation, and therefore an estimate.

I often tell people that the abbreviation of “estimate” is “guess.” I do this to remind people that they’re just estimates, not data. When observations and estimates disagree, you’d be prudent to trust the observations. When you don’t yet have any confirming or disproving observations, you should think about how much trust you put into the estimate. And think about how much risk you have if the estimate does not predict reality.

This does not mean, however, that you have to estimate by guessing. There are lots of ways to make an estimate more trustworthy.

Using more people to independently estimate is one common technique and provides a reasonableness check on the result. Wideband delphi techniques further this by then re-estimating until the predictions converge (or stalemate). People have widely adapted James Grenning’s “planning poker” to perform this procedure. In theory, having multiple independent estimates misses fewer important points and gives us a more trustworthy result.

In practice, the various estimates are often...

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