One thing that I get sick of is how people take an estimate, expect it to be accurate, feed it into a plan, and when the plan is (inevitably) proven wrong, blame the estimate. This is a fallacy from the era of Waterfall development, but it still lingers on in many Agile environments.
Here’s a scenario: take a team with 10 people in it. This means, in theory, you get 50 person-days a week of work out of the week.
A project comes along. It’s estimated that it will take about 1300 person-days, or about six months, to build, test, and implement all the various stages (naturally, as it’s an Agile project, there will be staged releases as well as incremental and iterative work). Great! Plug that into a project plan as a six-month project. Oh, and line up other things around it to make it important that it takes six months – no more, no less.
Except that’s a load of crap. Consider this: in Australia, it’s usual for every person to get 4 weeks of leave per year (which, on average, they take all of it eventually), and 2 weeks of sick leave per year (which, on average, they take about half). So, for that six month, or 26-week project, there will be, on average, 2.5 weeks added simply due to expected leave being taken. So that 26 week project should be expected to take 28.5 weeks – on average.
The real problem here is the amount of variation available. People may take no leave in the time of the project, or they may take more than they accrue (most places allow you to accrue up to 8 weeks of rec leave; then there is long service leave…). You can’t predict it at the start of the project, as people don’t have to give more than one month’s worth of notice. So all of a sudden, my project has gone from a simple 26 weeks to a complex 28.5 weeks, with a lower bound of 26 to an upper bound of about 42 weeks (every person starts with 8 weeks leave, uses it, uses the two weeks sick leave, uses the next two weeks gained when the project goes past New Years, and uses the rec leave accrued during the project). All from just one variable that is out of the control of the project planner. And all this assumes the estimate is accurate in the first place.
There are an amazing amount of variables, most with a large range, that need to be tracked for project planning. Expecting an estimate made several months earlier to be accurate is crazy – but people still do it.