Two possible calculation methods
midpoint method
This method is easy to calculate, but it does not take into account headcount variations during the period.
2. Precise method (Reflect)
The denominator is the average headcount; it corresponds to the average of daily headcounts over the period. This method therefore smooths variations and yields a more accurate rate.
Impact and example
The attrition rate can vary slightly depending on the calculation method used. Let's take a detailed numerical example:
Concrete case: Assumptions
Period: 01/09/2025 → 30/09/2025 (30 calendar days).
Headcount on 01/09/2025: 100.
2 departures (last working day) on 06/09 and 11/09.
1 arrival on 25/09.
Daily headcount trajectory
Days 1–4: 100 (4 days)
Days 5–9: 99 (5 days)
Days 10–24: 98 (15 days)
Days 25–30: 99 (6 days)
Calculation details
midpoint method : (B + E) / 2 = (100 + 99) / 2 = 99.5
The attrition rate is therefore 100 * 2 / 99.5 = 2.01%
Reflect method : Hm = 2959 / 30 = 98.63
The attrition rate is therefore 100 * 2 / 98.63 = 2.03%
The result is therefore slightly different for September 2025. Indeed, with the Reflect method, the variation (in our case: a decrease) in headcount during the period is more apparent.