Software Estimation: What you're doing wrong

During the beginning of my career as a software development engineer one of things that I dreaded the most was the beginning of any project where I knew I was going to be asked to give an estimate for the software I was going to write. Often I thought "how can I estimate something I have no clue how I'm even going to build?"

Ultimately the dread I felt came down to this one simple fact. I had no clue how you could accurately estimate software. Over the years, as I've learned to estimate software development much more accurately, I've learned that what I thought of as software estimation was built on several assumptions that were incorrect.

Fallacies of Estimation

• Estimates require giving a specific date
• Estimates should be given on the spot
• Estimates can't change

Estimates Require Giving a Specific Date

This fallacy was probably the heart of the anxiety I felt about software estimation. I always assumed that an estimate was a fixed amount of time like 2 days or 1 month. It never occurred to me that an estimate could be a range. Fixed dates are rigid and inflexible and are easy to get wrong. Fixed dates cannot account for the fact that there are unknowns associated with the project. 

An accurate estimate will be a range. Ranges are more flexible and can help account for the unknown and any risks associated with the project.

Estimates should be given on the spot

This fallacy is based on the assumption that some big decision needs to be made right now and doesn't have time to wait for you to think about what unknowns you're dealing with. First off, if you're making a big decision on the spot based on a back of the hand estimate you shouldn't be in your role and shouldn't have the responsibility that you do. It's careless and dangerous to you, your product, and your customers to expect some engineer to accurately give you an estimate on the spot.

An accurate estimate will require you to do some research.

Estimates can't change

It may be true that the original estimate needs to be something that you can make a big decision or commitment based on. It's not true however that estimation stops once that decision is made or a project is put into motion. An estimate needs to be a living thing.

Estimates are about approximations. They're about dealing with ambiguity. As things become less ambiguous your estimate should be refined.

Where to go from here?

Now that we know the fallacies of software estimation we can define the characteristics of an accurate software estimate. An accurate estimate will:

• Give you an understanding of risk and unknowns.
• Quantify the known work.
• Be something that you can base a big decision on.
• Be refined as new information becomes available.
• Be a range with a 90% confidence interval.

In my next post I'll talk about the truths of software estimation and give you a set of resources to help you create accurate software estimations.