# Statistics/Summary/Averages/Harmonic Mean

### Harmonic Mean

The arithmetic mean cannot be used when we want to average quantities such as speed.

Consider the example below:

Example 1: The distance from my house to town is 40 km. I drove to town at a speed of 40 km per hour and returned home at a speed of 80 km per hour. What was my average speed for the whole trip?.

Solution: If we just took the arithmetic mean of the two speeds I drove at, we would get 60 km per hour. This isn't the correct average speed, however: it ignores the fact that I drove at 40 km per hour for twice as long as I drove at 80 km per hour. To find the correct average speed, we must instead calculate the harmonic mean.

For two quantities A and B, the harmonic mean is given by: ${\displaystyle {\frac {2}{{\frac {1}{A}}+{\frac {1}{B}}}}}$

This can be simplified by adding in the denominator and multiplying by the reciprocal: ${\displaystyle {\frac {2}{{\frac {1}{A}}+{\frac {1}{B}}}}={\frac {2}{\frac {B+A}{AB}}}={\frac {2AB}{A+B}}}$

For N quantities: A, B, C......

Harmonic mean = ${\displaystyle {\frac {N}{{\frac {1}{A}}+{\frac {1}{B}}+{\frac {1}{C}}+\ldots }}}$

Let us try out the formula above on our example:

Harmonic mean = ${\displaystyle {\frac {2AB}{A+B}}}$

Our values are A = 40, B = 80. Therefore, harmonic mean ${\displaystyle ={\frac {2\times 40\times 80}{40+80}}={\frac {6400}{120}}\approx 53.333}$

Is this result correct? We can verify it. In the example above, the distance between the two towns is 40 km. So the trip from A to B at a speed of 40 km will take 1 hour. The trip from B to A at a speed to 80 km will take 0.5 hours. The total time taken for the round distance (80 km) will be 1.5 hours. The average speed will then be ${\displaystyle {\frac {80}{1.5}}\approx }$  53.33 km/hour.

The harmonic mean also has physical significance.