Being able to use measures of data, make calculations, and explore relationships is an essential geographical skill.

Calculating percentage increase is an important skill for geographers to have. When geographers collect data over a period of time, the results may increase. Calculating a percentage increase allows a geographer to see how much their data has changed. For example, it may be useful to find out how much the width of a river channel increases as you travel downstream.

Method:

- work out the difference between the two numbers being compared
- divide the increase by the original number and multiply the answer by 100

**In summary: percentage increase = increase ÷ original number × 100**

For example: the number of robins in a woodland area are counted over two different months. In December 15 robins were counted. In January 23 robins were counted. What is the percentage increase of robins in the woodland?

The difference between the two numbers is 8.

8 ÷ 15 × 100 = 53.3

The percentage increase of robins found in the woodland is: 53.3%

Calculating percentage decrease is an important skill for geographers to have. For example it may be useful to find out how much the load particle size decreases in a river as you travel downstream.

Method:

- work out the difference between the two numbers being compared
- divide the decrease by the original number and multiply the answer by 100

**In summary, percentage decrease = decrease ÷ original number × 100**

For example: the number of robins in a woodland area in February and March are counted. In February 22 robins were counted. In March 12 robins were counted. What is the percentage decrease of robins in the woodland?

The difference between the two numbers is 10.

10 ÷ 22 × 100 = 45.4

The percentage decrease of robins found in the woodland is: 45.4%

**Percentiles** and **quartiles** are both ways of dividing data into smaller parts. Whereas **quartiles** divide a set of data into **four equal parts**, **percentiles** divide the set of data into **100 equal parts**.

Percentiles are commonly used to plot the growth of babies. For example, a midwife weighs baby Anna and she is in the 90th percentile. This means that if there were 100 babies (of the same age), 90% of them would weigh the same, or less than baby Anna and 10% would weigh more.