# Adding Estimates

I’ve talked about estimation many times before. I’ve talked about Kent Beck, and I’ve talked about Hillel Wayne. Today I’m going to talk about all three of them at once. I’m not sure how directly this applies to development, but I’m pretty sure there’s something there. Maybe I’ll have it by the end of the post.

Beck recently wrote about private estimates and public progress. What it basically came down to was that the **entire** team gets together on Monday and decides what they think they can get done that week, given all they know of the situation, the priorities, and the goals. Then, at the end of the week, the team gets together again, looks at what was done, then answers the question “How did we do?”.^{1} It’s a simple idea. He then goes on to give a list of pros you get from doing that and some things to avoid.

You get limits. You know what you’re **NOT** doing. You know what done looks like. You get focus and alignment.

On the other hand, you can’t add weeks. Or compare them. Or compare teams. It just doesn’t work.

But it also got me to thinking about something Hillel said. You can’t add times or temperatures and get something meaningful, but you can subtract them and average them. Huh? On the surface, that doesn’t make any sense. I mean it’s true, and it feels right, but huh? Subtraction is just the inverse of addition. Why can you do one and not the other? Averaging is adding things up then dividing. So why can you add and divide, but not just add?

As he explains, the reason it works is that what you’re really doing is not staying in the domain of time or temperature. What you really end up doing is working in the domain of deltas in time or temperature. And that works because you can add and subtract deltas. When you add two times the result isn’t a time. I’m not sure what it is, but it’s not a point in time. When you subtract two times you get the difference between them. The delta. And I know what that is. It’s a time period. That’s a real thing.

Taking the average of a group of times (or temperatures) is really taking the delta between each time and a common arbitrary time point, then adding the *deltas*, dividing that time period by the number of times you’re averaging, then adding that time period back to the same arbitrary time point. All of those are valid operations, and you end up with a single point in time that we all agree is the average of that collection of time points.

Now that I’m getting towards the end, here’s what I think. Estimates are kind of like times and temperatures in that they’re based on a known starting point. If you could somehow compare them all with the same starting condition then, just like time or temperature, you could find the delta and do your math in that space. Unfortunately, the starting point for an estimate isn’t an estimate, or a point in time, or anything else easily measurable. Instead, it’s the sum total of what you know about a situation. How things are, how you want them to be, the environment around you, and the expected interruptions among other things.

When you make an estimate for **this week** you know everything you know about the current state. How correct you are is a different question, but you know all you can know right now. If you could know more then you would. However, the estimate you want to make for *next* week relies on having that same level of knowledge. You might have that knowledge now, but when you get to the beginning of next week, when you *should* be making that estimate, the current state will be different. How different? I don’t know. You don’t know.

For next week, it will *probably* be something like what you estimate for this week, but not exactly. And over time the difference between you expectation and reality will grow, to the point where you have no idea what the starting condition is.

Which mean that you can’t pick a starting condition to subtract from all of the estimates, so you can’t generate a relative estimate delta. And if you can’t turn your estimate into a delta then there’s nothing you can add, or even compare.^{2}

The same thing applies to comparing teams. Or doesn’t apply. Because the teams are different there **can’t** be any shared arbitrary point to use to create a delta from. And again, without that delta, there’s nothing to compare.

And that’s how Kent Beck, Hillel Wayne, and Estimates come together to explain why you can’t just add up a bunch of weekly estimates and come up with 12 month plan.