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Predictability is the ratio of completed to committed work, but the useful companion is velocity variability. A team averaging 28 points with a standard deviation of 5 can only commit to about 22 points if it wants to hit the commitment nine sprints in ten. Committing to the mean guarantees missing half the time, which is what erodes trust in estimates.
Predictability
say-do ratio = completed / committed; coefficient of variation = sd / mean; 90% commitment = mean - 1.28 x sd
Consistently hitting 100 percent usually means padding. Something in the 85 to 95 percent range with low variability is healthier than a perfect record.
Because planning depends on predictability. A slower but consistent team is easier to plan around than a fast but erratic one.