Currently we are working in sprint 5. You (or we) did not complete your sprint at time :( There was this one nasty story that appeared so easy but was in the end gigantic complex. Your team say they finished 50% of the story and the story was estimated 8 points. Your goal was to complete 30 points in total but you only finished 22 points in sprint 5.
- You velocity was not 26 points (22 points finished + 4 points partly finished). You did not earn 4 points by doing 50% of the 8 points story. You only receive points for stories you finish. So your velocity in sprint 5 is 22.
- The estimation of 50% finished of the 8 points story is probably just a wild guess of your team. For some reason the original estimation of 8 was already wrong, so there must be something wrong with the estimation. Perhaps the story is too big to make a good estimation?
- Your team did learn that 30 points was perhaps to much to handle. (You did not learn that 22 is your maximum amount of points)
In this example our team gathered together in the next sprint planning and I asked them "how many points it would take to finish this story taken everything (new knowledge, work already did) into account". The team estimated the specific story 6 points to finish. 2 less that the original estimation (8 points) and 2 more than what they initial thought. They learned much about the story in the last sprint and, after the new discussion, they found out they do not did finish 50% but only 25%. The sprint planning was ideal to have the discussion about the story to get new insights.
So what about the work in Sprint 5? What about the 50% of the 8 points story? Where are our 4 points? Well nowhere really! They are just lost. There is no artefact in Scrum where you report unfinished work. And that is completely ok!
You should realize that in Scrum much of the reporting is seen as waste. When you want to know how much progress you make, you look at finished (releasable) stories, not at the amount of points you have finished. Points and velocity are tools for a team to help them to be predictable about the amount of work they can finish. They are just tools! It is not a reporting tool for progress. Keeping that in mind makes losing points not really a problem anymore, does it?
On top of that there is another argument not to cheat with points. Pretending you did finish 4 points (50% of the 8 points) will actually stop you from learning about your possible velocity at all! If velocity means what your team can finish, adding points of unfinished stories doesn't make sense.
So, do not cheat with unfinished points. Cheating will not help you be more predictable, it is in fact counterproductive for your process. Hope this helps you to kill that ever lasting discussion about the lost points :)