Harmonic mean

In mathematics, the harmonic mean is one of several methods of calculating an average. Typically, it is appropriate for situations when the average of rates is desired.

The harmonic mean ($$H$$) of the positive real numbers a1,...,an is defined to be


 * $$H = \frac{n}{\frac{1}{a_1} + \frac{1}{a_2} + ... + \frac{1}{a_n}}.$$

Examples
In certain situations, the harmonic mean provides the correct notion of "average". For instance, if for half the distance of a trip you travel at 40 miles per hour and for the other half of the distance you travel at 60 miles per hour, then your average speed for the trip is given by the harmonic mean of 40 and 60, which is 48; that is, the total amount of time for the trip is the same as if you traveled the entire trip at 48 miles per hour. (Note however that if you had traveled for half the time at one speed and the other half at another the arithmetic mean, 50 miles per hour, would provide the correct notion of "average".)

Similarly, if in an electrical circuit you have two resistors connected in parallel, one with 40 ohms and the other with 60 ohms, then the average resistance of the two resistors is 48 ohms; that is, the total resistance of the circuit is the same as it would be if each of the two resistors were replaced by a 48-ohm resistor. (This is not to be confused with their equivalent resistance, 24 ohm, which is the resistance needed for a single resistor to replace the two resistors at once.)

Harmonic mean of two numbers
When dealing with just two numbers, an equivalent, sometimes more convenient, formula of their harmonic mean is given by:


 * $$H = \frac {{2} {a_1} {a_2}} {{a_1} + {a_2}}.$$

In this case, their harmonic mean is related to their arithmetic mean,


 * $$A = \frac {{a_1} + {a_2}} {2},$$

and their geometric mean,


 * $$G = \sqrt[2] {{a_1} \cdot {a_2}},$$

by


 * $$H = \frac {G^2} {A}.$$

Relationship with other means
The harmonic mean is one of the Pythagorean means and is never larger than the geometric mean or the arithmetic mean (the other two Pythagorean means).

It is the special case $$M_{-1}$$ of the power mean.

It is equivalent to a weighted arithmetic mean with each value's weight being the reciprocal of the value.

Other names
In older literature, it is sometimes called the subcontrary mean.