We prefer to use relative, effort-based units for measurement, instead of time-based units such as man-hours. It is so, because when you start using man-hours, someone will be there to tell you: you’ve created 10 Product Backlog items this Sprint worth of 381 man-hours. You’re 6 people working in two-week sprints, which is 528 man-hours. Why is your output so low? What’s wrong?
We don’t want it, because as soon as the developers are blamed, they will start adding safety margins to their estimates. Instead of saying that something requires 20 man-hours, they will say it takes 35 man-hours to avoid being questioned. This creates a lot of problems. For starters, the student syndrome: work expands to fill in the available time. When they add margins, they automatically produce less.
Alright, that’s why we don’t like time-based units. In return, we use relative, effort-based units: story points. Do you know what it is?