Industry & companies
Attrition
The rate at which participants leave a company each year.
Also: churn, dropout rate
Attrition is the share of participants who stop within a period, usually a year. Companies rarely publish it, but some disclose the number of participants at the start and end of a year, and public companies sometimes report "active members" over time.
High attrition means a plan needs constant recruiting to stand still, which is why recruiting bonuses tend to dominate.