You can calibrate your estimations skills. This means that if you say, "there's a 50 % chance it is done before this day", 20 times, then in 10 of them it is done by that day, and in 10 of them it gets done later than that day.
Perhaps more usefully, it also means that if you say "there's a 90 % chance it is done before this day" then 18 times out of 20 will it be done by that day, and only around two times will it be late.
I have calibrated my estimation skill, so whenever someone challenges my estimation, I urge them to keep score and find out that I'm right just as often as I claim to be.
(If I'm particularly bold, I suggest a bet. I'm confident enough to make money on it in the long run.)
However, usually it doesn't get that far. Once I start explaining that it's a probabilistic measure of high certainty customers and managers back off.
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Some people want to be reassured that I don't plan on taking as long as my 90 % estimation suggests, so then I give them something like a 10 %--90 % span and say, "it's unlikely to be done before this day, but also unlikely to be later than this. This interval is wide today because there are many uncertainties for me. As I learn more about the problem, this interval will narrow."
In some implementations of Agile this is supposed to be the point of the team velocity. Not that you make the engineers change the way they feel about tasks, but instead discover the hidden rate of delivery.
That's also a way to do it! I prefer making it explicit at the lowest level -- it's nice to be able to ask someone for a probability and know that it's accurate. It also helps them grow as people and professionals.
Perhaps more usefully, it also means that if you say "there's a 90 % chance it is done before this day" then 18 times out of 20 will it be done by that day, and only around two times will it be late.
I have calibrated my estimation skill, so whenever someone challenges my estimation, I urge them to keep score and find out that I'm right just as often as I claim to be.
(If I'm particularly bold, I suggest a bet. I'm confident enough to make money on it in the long run.)
However, usually it doesn't get that far. Once I start explaining that it's a probabilistic measure of high certainty customers and managers back off.
----
Some people want to be reassured that I don't plan on taking as long as my 90 % estimation suggests, so then I give them something like a 10 %--90 % span and say, "it's unlikely to be done before this day, but also unlikely to be later than this. This interval is wide today because there are many uncertainties for me. As I learn more about the problem, this interval will narrow."