In mathematics, the
geometric mean is a type of mea or average,
which indicates the central tendency or typical value of a set of
numbers by using the product of their values (as opposed to the arithmetic mean which uses their sum). The geometric mean is defined as the
nth root of the product of n numbers.
For instance, the geometric mean of two numbers, say 2 and 8, is just the square root of their product; that is
. As another example, the geometric mean of the three numbers 4, 1, and 1/32 is the cube root of their product (1/8), which is 1/2; that is
![\sqrt[3]{4\cdot 1\cdot 1/32}=1/2](http://upload.wikimedia.org/math/b/4/3/b43d87baacd060b973c619b3b23a8a18.png)
.
Three-dimensional plot showing the values of the geometric mean of varying
x and
y values.
Three-dimensional plot showing the values of the geometric mean of varying
x and
y values.
A geometric mean is often used when comparing different items –
finding a single "figure of merit" for these items – when each item has
multiple properties that have different numeric ranges.
[1]
For example, the geometric mean can give a meaningful "average" to
compare two companies which are each rated at 0 to 5 for their
environmental sustainability, and are rated at 0 to 100 for their
financial viability. If an arithmetic mean were used instead of a
geometric mean, the financial viability is given more weight because its
numeric range is larger- so a small percentage change in the financial
rating (e.g. going from 80 to 90) makes a much larger difference in the
arithmetic mean than a large percentage change in environmental
sustainability (e.g. going from 2 to 5). The use of a geometric mean
"normalizes" the ranges being averaged, so that no range dominates the
weighting, and a given percentage change in any of the properties has
the same effect on the geometric mean. So, a 20% change in environmental
sustainability from 4 to 4.8 has the same effect on the geometric mean
as a 20% change in financial viability from 60 to 72.
The geometric mean can be understood in terms of geometry. The geometric mean of two numbers,
and
, is the length of one side of a square whose area is equal to the area of a rectangle with sides of lengths
and
. Similarly, the geometric mean of three numbers,
,
, and
, is the length of one side of a cube whose volume is the same as that of a cuboid with sides whose lengths are equal to the three given numbers.
The geometric mean applies only to positive numbers.
[2]
It is also often used for a set of numbers whose values are meant to be
multiplied together or are exponential in nature, such as data on the
growth of the human population or interest rates of a financial investment.
The geometric mean is also one of the three classical Pythagorean means, together with the aforementioned arithmetic mean and the harmonic mean.
For all positive data sets containing at least one pair of unequal
values, the harmonic mean is always the least of the three means, while
the arithmetic mean is always the greatest of the three and the
geometric mean is always in between (see Inequality of arithmetic and geometric means.)
