We've used Planning Poker for over a year now. Our experience is that it is a great and easy way of coming up with estimates. If you don't know what it is, you can read more about it here.
When we started it seemed natural to us then that the values of the Planning Poker deck represented hours; 1,2,3,5,8 hours and so on. We had a rule of thumb that said that no task/requirement/story should be larger than e.g. 18-24 hours. If estimators felt like playing a higher card than that then it indicated that the story needed to be split up into smaller bits.
So, this seemed fine, especially on paper. The problem was that many sessions became extremely lengthy and we often got stuck in discussions about details and solutions. I know this is to be avoided in Planning Poker, but that's what I wanted to point out here: we made that mistake. It wasn't as easy as just saying "No let's not talk about design details now"... We said that. Over and over :-). People felt they needed to discuss the details, in order to decide between e.g. a 5 or an 8, or between 8 and 13... It was often possible to force a play and ask people to follow gut-feeling and intuition first, but the discussions kept popping up over and over again.
Our approach on avoiding this and speeding up the sessions was to simplify: forget hours. We estimate days instead. Our smallest estimate now is 0.5 (days) and then we follow the typical Planning Poker deck series; 1,2,3,5,8 and so on. A typical task/story takes between 0.5 and 5 days, I would say.
In my experience estimating in hours causes a false sense of security and leads to over-confidence in the plans. One of the arguments against using days as the estimation unit may be that we then miss the opportunity of really thinking through what parts/activities/tasks a certain story/task consists of and thereby miss possibly important aspects in the estimate... Maybe it is so. I dont know yet... So far I think it's a good tradeoff; I trade waste of time and false sense of security for a possible decreased accuracy... Well, I've already accepted to sacrifice accuracy given the 2-3 minutes per story and the whole idea behind Poker Planning to begin with: a fast and simlpe way of estimating.
My understanding of the common practice in Scrum is that you estimate two sorts of things;
1. Estimate stories in Story Points, which is a relative estimate of complexity (regardless of time)
2. Break stories down into tasks and estimate tasks in hours.
Our implementation (so far :-)) is to simply estimate stories using Gnome Days. Sometimes we have to break stories down into tasks during a sprint planning session, but certainly not always. If we do, we obviously estimate the tasks individually, but we still use days as the unit, not hours.
I'm curious to hear about other people's experience in estimating days vs hours in Scrum.
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