Mean
What Is the Mean?
The mean is an average of a set of numbers.
The mean is found by adding all the numbers together and dividing by how many numbers there are.
Dictionary Definition
The Oxford English Dictionary defines the mean as "the average of a set of numerical values, as calculated by adding them together and dividing by the number of terms in the set."
Finding the Mean
Imagine we wanted to find the mean of the numbers given below:
To find the mean, add the numbers (1, 2, 3, 4 and 5) together and divide by how many numbers there are (5):
Mean = (1 + 2 + 3 + 4 + 5) ÷ 5
Mean = 15 ÷ 5
Mean = 3
Understanding the Mean
An average is a single value that is typical for a set of values. It is a value that can represent, or stand in, for all the values in a set.
In the example above, we found that the mean of 1, 2, 3, 4, 5 is 3.
3 is a typical of these numbers, it is somewhere near the middle of them. It is also representative of the numbers.
Imagine that the numbers in our example represent the amount of money that 5 friends have:
The mean amount of money is $3. It is the amount each would have if the friends added all their money together and shared it equally among the 5 of them:
This is a useful way of understanding the mean. It is the total of all the items shared out equally amongst each item.
A Formula for the Mean
Imagine we are finding the mean of n numbers, where n is a number. In the example below, there are 5 numbers, so n = 5:
We label each number in a set x_{i}, where i = 1 for the 1^{st} number, 2 for the 2^{nd} number, up to n for the n^{th} number. (In our example, n = 5, so x_{5} is the final number). This is shown below:
The formula for finding the mean is shown below:
In this formula,

x̄ is the symbol for the mean. It is said "x bar".

Σx_{i} means "sum of x_{i}" from i = 1 (below the Σ) to n (above the Σ).
Σx_{i} = x_{1} + x_{2} + ... + x_{n}
In our example, n = 5
Σx_{i} = x_{1} + x_{2} + x_{3} + x_{4} + x_{5}
Σx_{i} = 1 + 2 + 3 + 4 + 5
Σx_{i} = 15

This is all written above a line, with n under it. This means divide by n.
The Sample Mean and the Population Mean
We often wish to take the average of a lot of numbers. For example, imagine you wished to find the average income in a country.
To find the mean, we would first have to find the income of everybody who lives in the country.
The mean is found by adding all the numbers together and dividing by how many numbers there are.
We would call everybody's income the population. The population is all the numbers in a set for which we are trying to find the average.
The population mean would be the average of everybody's income. It is denoted μ.
However, finding every number in the population is very timeconsuming and may not be possible.
Instead, we choose a sample from the population.
We would find the incomes from a subset of the population. We would find a much smaller number of people (perhaps several hundred) who are representative of the total population and find their incomes.
We would then find the average of these numbers, finding the sample mean, which is denoted x̄.
We use the sample mean to estimate the population mean.