Statistical Glossary
Harmonic Mean:
Harmonic mean is a measure of central location. The harmonic mean of

Let the path between two cities
The harmonic mean is sensitive to a single small value 
For this reason, the harmonic mean is often used, for example, to aggregate scores in different types of activity to a single final score, e.g. to estimate students´ performance. This ensure that no partial scores are radically lower than the final score.
See also Mean values (comparison) .