Experiments to test a hypothesis should consist of at least two parts.

• A test where one variable is changed.

• A control where that variable is unchanged.

All other factors that may affect the experiment should be kept the same.

In our hypothesis of exposure to UV-B, a test group may be exposed to high levels of UV-B, while a control group suffers no such exposure.

All other factors, such as age, sex and diet, should be kept the same.