The problem with the word “estimate” is that it’s personal. It’s relative to the person who is estimating. Moreover, it’s a unit of time. That is why I prefer “sizing” stories rather than estimating them. Size is a non disputed (standard) unit, which measures the level of complexity involved in developing a piece of functionality, a story. The size is standard across a particular project team.
Pizzas are a great example of standard size. They come in small, medium and large sizes and, given the context of the vendor, the whole world knows what a small pizza means.
Pizza Appetite is the capacity to consume pizzas. A 5 year old will find a small pizza to big to eat. A teenager will probably find it perfectly filling. At 25, I find the small pizza too small. I can comfortably eat a medium pizza (or 2 small pizzas). I am sure there are people I don’t know who can eat nothing less than a large pizza (or 4 small pizzas)!!
Stories are like pizzas. Given the context of a project, they have fixed sizes. As a developer spends more time on a project, getting older (and wiser), his/her capacity to develop complex stories increases. And this is where the trouble starts. At the beginning of the project the developer could do 1 small story every day. But now, instead of realizing that he can do a medium story (or 2 small stories) in a day, he begins to think that the medium story is actually a small one. The thing to remember is that the size of the story remains the same. It’s just that the developer can now do more complex stuff in a given time.
Complexity points are never to be estimated (in fact they can’t be estimated because they are not relative to individuals). In the OLAP terminology, complexity points are the fact. An estimate is a calculation you (as an individual) do when you are looking at them from the Time dimension.
So if you are estimating units of time, it makes sense for you to re-estimate so that you take into account your improved understanding of the system.
When you are sizing stories in complexity points be very careful not to fall in this trap. Re-sizing even sounds wrong anyway.