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.

Independent Variables

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.

Dependent Variables

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 you 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.

Controlled Variables

Experiments also have controlled variables.

Controlled variables are things that would have an effect on the dependent variable.

The experimenter 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."

Variable types

There are three main types of variable: categoric, discrete and continuous, and the type you are dealing with will affect how best to display your results.

Categoric variables have values that are basically labels. They are 'words' e.g. names of plants, elements or types of material.

To plot a graph of categoric results, you should choose to do a bar chart.

For example: If you investigate the effect of acid on different metals, e.g. copper, zinc and iron, the type of metal you are using is a categoric variable, so you should display the results in a bar chart.

Categoric variables can be 'ordered' but they are nowhere near as 'measurable' in the ordering as discrete and continuous variables are!

Discrete variables are numbers, but they can only take values with a fixed interval between them. For example the number of throws of a dice - you cannot have half a throw! Throws will always be whole numbers. Also the number of protons in a nucleus - you will never have 'part of a proton'.

Continuous variables are numeric variables that have an infinite number of values between any two values. A continuous variable is numeric. Volume, length, distance, current, time, acceleration - many things can have any value and therefore are continuous.


Independent Variable
Dependent Variables
Controlled Variables
How does a tap affect how much water flows through it?

Water tap opening (closed, half 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 your examining board will expect you to know several terms relating to variables.

Click here to look at a typical list of 'practical science vocabulary' that you are expected to know. But always look carefully at your own syllabus - and highlight 'technical' terms they use when writing it - they will use those terms in questions and expect you to use them in answers.

See here for a page on what type of graph to choose to display your results.