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Mean, Median, and Mode

The Mean of a Data Set

The mean of a set of numbers, sometimes simply called the average , is the sum of the data divided by the total number of data.

Example 1 :

Find the mean of the set { 2 , 5 , 5 , 6 , 8 , 8 , 9 , 11 } .

There are 8 numbers in the set. Add them all, and then divide by 8 .

2 + 5 + 5 + 6 + 8 + 8 + 9 + 11 8 = 54 8 = 6.75

So, the mean is 6.75 .

The Median of a Data Set

The median of a set of numbers is the middle number in the set (after the numbers have been arranged from least to greatest) -- or, if there are an even number of data, the median is the average of the middle two numbers.

Example 1 :

Find the median of the set { 2 , 5 , 8 , 11 , 16 , 21 , 30 } .

There are 7 numbers in the set, and they are arranged in ascending order.  The middle number (the 4 th one in the list) is 11 .  So, the median is 11 .

Example 2 :

Find the median of the set { 3 , 10 , 36 , 255 , 79 , 24 , 5 , 8 } .

First, arrange the numbers in ascending order.

{ 3 , 5 , 8 , 10 , 24 , 36 , 79 , 255 }

There are 8 numbers in the set -- an even number. So, find the average of the middle two numbers, 10 and 24 .

10 + 24 2 = 34 2 = 17

So, the median is 17 .

The Mode of a Data Set

The mode of a set of numbers is the number which occurs most often.

Example 1 :

Find the mode of the set { 2 , 3 , 5 , 5 , 7 , 9 , 9 , 9 , 10 , 12 } .

2 , 3 , 7 , 10 and 12 each occur once.

5 occurs twice and 9 occurs three times.

So, 9 is the mode.

Example 2 :

Find the mode of the set { 2 , 5 , 5 , 6 , 8 , 8 , 9 , 11 } .

In this case, there are two modes -- 5 and 8 both occur twice, whereas the other numbers only occur once.