Accuracy versus Precision

Some people might think accuracy and precision are one and the same, or at least very closely related. But although these words are often used interchangeably, even in dictionaries, they do not always mean the same thing.

Estimating for Accuracy

In the business of estimating, these differences are especially apparent. Precision in estimating reflects a level of detail or granularity. Accuracy is simply the quality of being correct. Given these two definitions it is possible to be very precise without being accurate, detailed without being correct.

That's because estimates by definition involve uncertainty, being cast in the future. If they were certain, they would be in the past, and we would refer to them as statistics or logs.

Estimates can likewise be very accurate without being very precise. Sound backwards? No, estimates that are lucky guesses don't count.

Precision and Realism

In the simplest terms, an accurate estimate need only be a statement that is true, regardless of its form. Thus, the more precise the estimate, the harder it will be to make an accurate statement.

For example, if my estimate states "the project will cost more than zero dollars," then I have a really good chance of being right. And if I get brave and do my homework and say "We can upgrade your system for between $140,000 and $160,000," my chance of success is still really good. If I say, "we can do it for under $160,000", my chance of success is likewise good. But as soon as I say "We can build your project for exactly $151,298.42," then any sane client would think I was smoking something.

Conclusion

So if accuracy is really your priority, then set aside the trap of too much precision. Use ranges instead of absolutes. Be honest about the fact that you can't possibly know all the contingencies up front. Be realistic in your numbers, and you can avoid the pitfalls that come with making promises you can't keep.

-Tom/*