A weighted harmonic mean combines positive rates by adding the exposure that each rate covers. It is designed for situations where the rate sits in the denominator.
The relationship
The defining relationship
Hw = Σwi ÷ Σ(wi ÷ xi)
The positive xi are rates; wi represent matching numerator-side exposure, such as distance when averaging speed.
See the idea
Equal distance means add times first
60 km/h1 km takes 1/60 h
40 km/h1 km takes 1/40 h
2 km ÷ (1/60 + 1/40) h = 48 km/h
Worked example
Two equal-distance travel legs
At 60 km/h and 40 km/h over equal distances, the average speed is 2 ÷ (1/60 + 1/40) = 48 km/h, not 50 km/h.
Interpret with care
Choose the relationship that fits
The weights must reflect the right exposure. For speeds observed for equal time instead, the arithmetic mean is the relevant relationship.