Finding an average is incredibly useful when you want to find one piece of data that describe a whole set of data.

You will already know that if I wanted to find the average from a list of numbers, I would just add the numbers and then divide by how many there were.

**What if the list is really, really long?**

Imagine that you collected the shoe size of 500 people. Would you want to add them all together?

There has to be a quicker way.

**Using a frequency table.**

If we have loads of data, we would normally put it in a frequency table. Let's look at an example.

**Example 1: I collect the shoe sizes of a group of people and put it into a frequency table. What is the mean of the shoe sizes?**

Shoe Size | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |

Number of people | 5 | 7 | 9 | 14 | 21 | 28 | 16 | 6 | 5 | 1 |

Let's look at the first column. This means 5 people have a shoe size of 3. If I wrote this as a list I would have 3,3,3,3,3 (Adds to 15)

The second column means that 7 people have a shoe size of 4. If I wrote this as a list I would have 4,4,4,4,4,4,4 (Adds to 28)

**Can you spot the link?**

To find the total, all I do to start with is multiply the two numbers together! If I did this for all of them, I would get...

Shoe Size | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |

Number of people | 5 | 7 | 9 | 14 | 21 | 28 | 16 | 6 | 5 | 1 |

15 | 28 | 45 | 84 | 147 | 224 | 144 | 60 | 55 | 12 |

Once we have this, we can work out what all the shoe sizes added together would be by adding the numbers in red. (814 )

We now have one of the bits of information I need to find the mean (what they all add up to)

All I need now is to find out how many people I asked. I can get this by adding up all the numbers in the number of people row. (112)

To find the mean, all I now have to do is to divide one of these numbers by the other (814 ÷ 112) to get an average of 7.3

**A couple of points.**

Does it matter that the answer is a decimal, you can't have a shoe size of 7.3.

No - It's not a real shoe, it just represents all the others, decimals are fine.

How do I know which way round to divide?

If you can't remember, try them both ways round. You'll get two answers (7.3 and 0.14).

Remember that the average must be in the middle(ish) of the data so it must be 7.3.

**Can you summarise for me?**

Step 1: Multiply the two columns together

Step 2: Add all these new numbers up

Step 3: Add up all the frequencies

Step 4: Divide to find the mean.