A guest blog from Dr Jenny Shipway, who studied biochemistry at university and now works in science communication and education training.

To understand experimental design and graphs, you will need to confidently recognise the difference between different types of variables. Master this now and it also will make it much easier to learn content that is taught using graphs.

What is a Variable

A variable is any value that does/might change during an experiment. Variables can include things like pH, temperature, colour, or the concentration of substances. The amount of time that has passed is a variable, as are rates of reaction.

Some variables’ values are fixed deliberately by the scientist.
Some variables’ values are changed deliberately by the scientist. These changes are planned in advance so the values are known before the experiment starts.
Other variables’ values are allowed to change naturally as the experiment progresses. These changes can be measured to provide useful data.

Simplify Your Exam Approach

During exams it’s very easy to get overwhelmed with information. When this happens, your brain makes guesses and jumps, often without you being aware. This is why students so often mis-read graphs and/or make ‘silly’ mistakes.

Excellent Exam Tip: work out what the experiment was and how the graph is presented BEFORE looking at the exam question.

This will …

Reduce the number of things you’re thinking about at one time
Reduce the risk of you jumping to (incorrect) conclusions
Help you focus on understanding the experiment

You might feel an urge to rush forward to look at the question, but trust me that won’t save time. You will not be able to answer the questions unless you understand the graph, and rushing forward will make that more difficult, not less.

Identifying Variables

Variables described in the experiment and on the graph axes may include:

Independent Variable – this is the thing that is changed deliberately by the scientist in a planned way. This is what we expect to cause a measured effect.
Dependent Variable- this is the thing that is measured by the scientist. The value is not known until it is measured, and the value will depend upon the value of the independent variable.
Control Variables – these variables are fixed to one unchanging value throughout. For example, all experiments might be carried out at 20°C, in which case temperature is a control variable.

An experiment is run to answer the question: how does the independent variable affect the dependent variable?

There are usually many factors that could affect the dependent variable, so it’s important to control as many other variables as possile. The idea is that if only one thing has changed, then you can be more confident the affect is due to that one thing.

An Example 🍅🍅🍅

If you wanted to work out what temperature was best for growing tomatoes, you could try growing plants at different temperatures to see which plant produced the most fruit. Everything except temperature should be kept the same so that you’re sure that any difference in fruit yield is due to temperature.

The question is: how does temperature affect fruit yield?

You will deliberately be changing the temperature to pre-chosen values, so temperature is the independent variable.

You will need to measure the weight of tomatoes produced to find out this value. So this is the dependent variable. The weight of fruit will depend upon the temperature.

Check your Understanding

Can you identify the independent and dependent variables in the following examples? Which variable was independently chosen by the scientist? And which was measured/recorded during the experiment?

(Ignore the positions of the data points, you only need to look at the axis labels.)

Usually, you will find the independent variable along the x-axis, and the dependent variable up the y-axis. But that’s not always the case (as you hopefully spotted in some of these examples). Do not let your brain jump to an assumption! You must always check this.

You need to be 100% confident of your variables before you move forward to look at the actual question, or everything else will get really confusing. It’s a good tip to write on the exam paper which variable is which (eg label them I.V. and D.V.)

Multiple Independent Variables

It’s possible to have more than one independent variable.

For example, some experiments are run twice under different conditions. See this graph:

First, look at the axes. The independent variable here is the amount of time that has passed (on the x-axis): the scientist decided before the experiment at what times they would count the fruit. The dependent variable is the number of ripe fruit (on the y-axis): the number of ripe fruit is the thing the scientist is measuring, and this depends on how much time has passed when the measurement is taken. We’ve seen this before.

Now look at the data. There are two sets of data plotted on the graph. One experiment has been run with fertiliser, and one without. So the presence of fertiliser is another independent variable – it’s something else that affects the value of the dependent variable.

Example A-level Exam Questions

Can you identify the independent and dependent variables in the following A level Biology exam questions?

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For more general information about graphs, see the post about how best to approach A level biology graph questions.