# What does the team's velocity mean?

Simple explanation is that team's velocity is the amount of work the team can accomplish in one sprint. But what does it actually mean if the teams velocity is 16 points. Does it mean that the team can not do 17 points in sprint in any circumstances? Or that 15 points will always succeed? What about 10 or 20?

And how can we know what the teams velocity is? Team might have successfully finished following amount of points in previous sprints: 25,20,11,11,9,23,15. What is the velocity? Average is about 16 points. This might sound reasonable velocity to start filling the sprint with issues with. However based on historical data this actually means that the team would fail half the time. That is the nature of the average.

Figure 1: Probability distribution of points done in iteration

Above is normal distribution is based on the average and standard deviation of the previous numbers. Just seeing the distribution does not help much. Cumulative distribution is much better tool in this case. If cumulative distribution is subtracted from 1.00 the resulting graph will describe how likely it is to get sprint done with certain amount of points.

Figure 2: Iteration size and success probability

I think it is vital to remember the probabilistic nature of team's velocity. It is OK to choose 15 points of issues to the sprint, if one can live with 40% likelihood of failure. However it is not a good idea to trust that 150 points would get done in 10 sprints. Every sprint has only 60% probability of success, succeeding in ten sprints straight has only 0.6% probability.

Of course this example should be taken with grain of salt. The actual shape of probability distribution for points done in sprint is not know. Also the calculations are based on historical data, which that might not be good indicator for future progress. Just by looking at the numbers one might miss out that holiday season or similar "catastrophe" is coming.