>And if the last few sprints they've completed between 5 and 30 points, do you believe they'll complete around 17.5 points next sprint?
I don't know, what did the team figure out in retro? Was the big difference a real underestimation, or was there some kind of unforeseen blocker? I've never seen that big a variation, but anything's possible.
If it makes you feel better to measure team velocity in something you call "days" instead of story points and it works for your team, more power to you. But don't fool yourself that you're talking about actual days. At best you're talking about "probable days", and how many days it actually takes will depend on a lot of things, including unknowns and who takes the story (are "Bob-days" the same as "Carol-days"?). So you'll end up with a measure of days that is very team- and uncertainty-dependent, and at that point it's better to just use story points and admit that it's not a universal measure and doesn't need to be. Not to mention that by using days you'll invite confusion between calendar time and time-on-task.
I don't know, what did the team figure out in retro? Was the big difference a real underestimation, or was there some kind of unforeseen blocker? I've never seen that big a variation, but anything's possible.
If it makes you feel better to measure team velocity in something you call "days" instead of story points and it works for your team, more power to you. But don't fool yourself that you're talking about actual days. At best you're talking about "probable days", and how many days it actually takes will depend on a lot of things, including unknowns and who takes the story (are "Bob-days" the same as "Carol-days"?). So you'll end up with a measure of days that is very team- and uncertainty-dependent, and at that point it's better to just use story points and admit that it's not a universal measure and doesn't need to be. Not to mention that by using days you'll invite confusion between calendar time and time-on-task.