# Calculations

## Calculating percentage increase

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

Calculating percentage decrease is an important skill for geographers to have. For example it may be useful to find out how much the 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

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.