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How Do You Accurately Estimate Programming Time?
itwbennett writes "It can take a fairly stable team of programmers as long as six months to get to a point where they're estimating programming time fairly close to actuals, says Suvro Upadhyaya, a Senior Software Engineer at Oracle. Accurately estimating programming time is a process of defining limitations, he says. The programmers' experience, domain knowledge, and speed vs. quality all come into play, and it is highly dependent upon the culture of the team/organization. Upadhyaya uses Scrum to estimate programming time. How do you do it?"
I'll throw together a script that takes in all of the project parameters as input, then based on the number of people in the team, past performance, scheduling deadlines and intermediate deliverables, comes up with an estimate for final delivery.
Then I throw out that number completely, and just multiply the time it took me to develop that script by five. This method has proven disturbingly accurate.
Scrum is a way of chunking development into well-defined portions. The idea of using Scrum to estimate time just doesn't make sense. Everything in Scrum takes the same amount of time. Two weeks. (Or one week, or whatever your sprint length is.) The difference is that long projects are implemented over multiple sprints, since obviously, not everything can be done in two weeks. So the estimate is not of how long it will take, but how many backlog items will be required in order to reach some known endpoint. Once the backlogs have been created and agreed upon by the team, estimating the necessary time becomes a matter of multiplication: 12 backlogs * 2 weeks = 24 weeks to finish this product.
This makes you shift your thinking from "how long will it take to do all these things" to "how can I break this product development into chunks which each fit into a two-week period?" That's much easier than making wild-ass guesses about the time it takes to do something.
The method I was taught in school, back in the days when Arthur Andersen existed, and prior to Andersen Consulting or AssVenture (or whatever they're called now), the method that was taught to me was credited to them, and looked like this:
Estimate how long it will take, if everything goes right. Call it T1.
Estimate the expected time it will take, knowing how some things usually go wrong. Call it T2.
Estimate the worst possible case, if everything goes wrong, and there are lots of unforeseen issues - you are 99% sure you can get it done within this amount of time. Call it T3.
The Weighted average estimate is (T1+(4*T2) + T3)/6
This yields some very odd T values, but often works out to be a reasonable estimate. The developers may tell me "If all goes well, it'll take 2 days. I expect it to be 4 days. But worst case, it might be 40 days." And you end up with a 9.6 day estimate. (which is SIGNIFICANTLY different than the "I expect it to be 4 days".)
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The part I learned from working for a major computer manufacturer was this:
Take whatever estimate the developers tells you, and tell the client to expect it in double that time (from today). If the lab tells you they'll have a fix by tomorrow, tell the client it'll be there the next day. If the lab tells you it'll be ready in three months, tell the customer 6 months. That way, as time goes by, the estimates become more real, and the variation from the estimate becomes more refined. And it accounts for the unavoidable time between when the lab releases the work, and when it gets installed at the customer's site.
If, in January, I was told by the developers that it'd be ready in 2 months, I will tell the customer to expect it in 4 months (May). If in February, I haven't gotten a revised time-frame, then we still expect it in four months (June).
Manage expectations, and don't let the customer down (particularly when things are outside my control).