What does that mean, between 200 and 400 hours? Just give me a number!
I know a whole bunch of developers in different companies and different businesses. What they all love most about their jobs, is when they are asked to produce an estimate. It’s the highlight of their week!
You’ve had a creative discussion with your boss and perhaps a couple of representatives from the customer. It’s all fun and games, until your boss turns to you and asks The Question.
“So, how long does it take?”
Did he just do that? In front of the customer, no less? How on earth are you supposed to answer that reliably without any time to analyse the details? You venture a somewhat vague answer, even though you know it’s no use.
“Hm, perhaps somewhere between 200 and 400 hours?”
“What does that mean, between 200 and 400 hours? Just give me a number!”
“Oh, uhm, I guess around 300 then?” you reply desperately, automatically reaching for the average of the first two numbers you threw out there. Surely that can’t be too much off?
Any one number representing a possible future outcome of something, will never be anything except just that. One possible outcome. That means somewhere out there, you’ve got a whole bunch of other possible outcomes as well. So If you say it might take 200 hours, without the backing of additional data, the chance of it taking more or less is about 50% either way.
So you’re giving your boss a definite number based on your guess on the outcome of something, without any information about other just as likely outcomes. Does that sound like a useful number to you? I didn’t think so.
Estimates are usually needed to figure out wether to invest in something, and to set a budget. If you give me a number where it’s a 50% chance of me blowing my budget, then I’d say you’re not really helping me over here. It’s heads or tails wether I’m in trouble with the customer for overspending. So when I ask for an estimate, it’s implied that I need a number that isn’t very likely to be too low.
So if your boss (or your customer) insists on getting that one number, what to do? First we need to understand more about the problem of “just give me a number”. More on that in part #2 on this mini-series about estimating!