Being Too Precise Hurts Your Plans - Mike Cohn

I need a new backup drive. I don’t need a 40 TB drive but the site I buy from listed one available. I thought I might as well click on it to see the price.
The price seemed reasonable for that much storage, but what caught my attention is that the drive ships in 205 days.
Seriously, what are they thinking? How can they possibly know the drive will ship in 205 days, not 204 or 206?
This website fell into what I call the precision trap, which is applying a false level of precision to some estimate or plan. We often fall into the precision trap because of the math involved in planning.
Let me clarify this with an example. Suppose a team has estimated that it needs to deliver 100 story points of work to achieve some objective. They’ve already calculated their velocity to be 15. Someone on the team does the simple math of 100 divided by 15 and gets 6.66. Team members then proclaim that they will be done in 6.66 sprints. Or hopefully someone decides to round that up and say it will take 7 sprints.
But math like this leads to the precision trap. You can see from this how the website decided the drive would ship in 205 days.The precision trap persists because we seem wired to like precision. It feels good to say we’ll be done in 6.66 sprints. We must be really smart to know that.
But we need to favor being accurate over being precise. Accuracy is about being right. The easiest way to be right is to be less precise.
For example, that website could have told me the drive would ship in “about 7 or 8 months.” That would have been enough precision for me to decide whether to buy it.
When math tells a team they can deliver in 6.66 sprints, that is very precise. But it’s probably not very accurate. Just like the ship date of the hard drive, that estimate should be conveyed as a range. Instead of 6.66 sprints, maybe it’s 6 to 9 sprints or even 7 to 10 sprints.
Avoid falling into the precision trap if you want to succeed with agile

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