Scientists use an experiment to search for cause and effect relationships in nature. In other words, they design an experiment so that changes to one thing causes something else to vary in a way that the scientist can describe as a 'trend'. The most useful way to describe a trend is a mathematical one.

These changing quantities are called variables, and an experiment usually has three main kinds: independent, dependent, and controlled.

The independent variable is the one that is changed by the scientist. In an experiment there is only one independent variable. This is usually plotted on the X-axis of the graph that the scientist uses to display his/her results in.

As the scientist changes the independent variable, he or she observes what happens.

The dependent variable changes in response to the change the scientist makes to the independent variable. The new value of the dependent variable is caused by and depends on the value of the independent variable. For example, if you turn on a water tap (the independent variable), the quantity of water flowing (dependent variable) changes in response - the water flow increases. The more open the tap - the faster the flow of water. The number of dependent variables in an experiment varies, and there is often more than one.

Experiments also have controlled variables. Controlled variables are things that would have an effect on the dependent variable. S/he must be sure that the only thing affecting  that variable is his/her adjustment to the independent variable.

So, controlled variables are quantities that a scientist needs to keep constant, and s/he must observe them as carefully as the dependent variables.

For example, if we want to measure how much water flow increases when we switch on a tap, it is important to make sure that the water pressure from the water supply (the controlled variable) is held constant. That's because both the water pressure and the opening of the tap valve have an impact on how much water flows. If we change both of them at the same time, we can't be sure how much of the change in water flow is because of the faucet opening and how much because of the water pressure.

Most experiments have more than one controlled variable. Some people refer to controlled variables as "constant variables."

Question Independent Variable Dependent Variables Controlled Variables Comments
How much water flows through a tap?

Water tap opening (closed, 1/2 open, fully open)

(an ordered categoric type of variable)

Volume of water flowing measured in liters per minute


(a continuous type of variable)

Water pressure (how much the water is "pushing")


(a continuous type of variable)

A better measure of the independent variable would be to find area of the opening in the pipe in square centimeters.
How fast does a candle burn?

Time measured in minutes



Height of candle measured in centimeters



  • Use same type of candle for every test
  • Wind--make sure there is none


In this case, time is what causes the dependent variable to change. The scientist simply starts the process, then observes and records data at regular intervals.
Does fertilizer make a plant grow bigger? Amount of fertilizer measured in grams
  • Growth of the plant measured by its height
  • Growth of the plant measured by the number of leaves

(categoric - discrete)

  • Same plants
  • Same soil
  • Same size pot
  • Same amount of water and light
  • Make measurements of growth at the same time

(categoric and continuous)

Does an electric motor turn faster if you increase the voltage?

Voltage of the electricity supplied to the motor measured in volts


Speed of rotation measured in RPMs


  • Same motor for every test
  • Same load on the motor



At GCSE the AQA examining board expect you to know several terms relating to varibales. Click here to look at the list of 'practical science vocabulary' that you are expected to know.