High level estimates should be provided in T-Shirt sizes. Doing so forces us to use a very low precision which avoids creating a misconception of accuracy in the quote. E.g. Saying it will take 328 person-days implies the accuracy in the last digit which means +/- 5 person-days. This level of accuracy is very rarely correct and involves extensive wasteful analysis to try and achieve it. Nor would a difference of +/- 5 person-days make a difference to the business decision making process.
When quoting T-Shirt sizes, our colleagues will still need to know tangible cost and time this roughly translates to so estimates can be used in decision making. To do this, the following guide can be used for quoting and explaining the cost and time.
It should be understood that we are putting the two variables of time and cost into one high level measurement. There will of course be combinations that don't fit. e.g. 2 months 10 people. In these cases still quote a T-Shirt size that is closest.