Science: what are experimental variables?

Short answer: they’re what will make your Independent Research Project a success or failure. Long answer: coming right up.

(Before we dive in here, have a read of this article so you know what we’re talking about when we say “Ron’s protein shakes”.)

Right, experimental variables. They’re the factors than can change in your experiment. Here’s an example, from Year 11 Biology, before we dig deep.

Question: “How does the surface area of a chemical substance affects the rate at which it dissolves in water?” Students will take three soluble tablets and break one in half, break one into quarters and leave the third tablet intact. Each tablet is placed in water and students time how long it takes to dissolve.

So far two experimental variables have been mentioned. They are:

The Independent Variable: The (A) Factor

This is what WE change to see what happens.

In the Year 11 tablet experiment, the Independent Variable is the surface area of the tablets. Students increase the surface area by breaking the tablets, which exposes the inside parts of the tablets to the water. As you may have hypothesised, this experiment always results in the tablet broken into quarters being the fastest to dissolve (or it should!).

In the experiment to see if Ron’s protein shake works, the Independent Variable is whether the participant is drinking shakes or not.

The Dependent Variable: The (B) Factor

This is what we are measuring or observing to see if changing (A) has any effect.

The Dependent Variable in the Year 11 tablet experiment is the time it takes for each tablet to dissolve, measured in seconds. Some experiments will involve observations only – a good example is using the “greenness” of leaves as an indicator of a plant’s health.

The Dependent Variable in the protein shake experiment is the difference in muscle gain (if any) between the twin brothers.

Once you have decided on your question – and your Independent and Dependent Variables – it’s time to make sure the test can be completed fairly. To do this you have to consider what must remain unchanged.

The Controlled Variables: The (C) Factor

These are things that may affect the result of a test and WE DON’T WANT THAT TO HAPPEN.

We must make sure all the other possible variables in an experiment remain unchanged. Going back to our Year 11 experiment, the rate at which the tablets dissolve in water can be affected by many things: water volume, temperature, impurities and unequal masses for the tablets. If you shake the water for one tablet but not the others, it’s going to make a big difference to your results.

To prove if Ron’s protein shake is effective, diet, exercise habits, sleeping habits and clothing are just of the four variables that would need to remain unchanged, with possibly many more that we haven’t considered.

Make your question a realistic one

Tell your teacher your idea for a question and discuss if it possible to achieve with simple equipment. Also, check if controlling all the variables is a realistic goal. Here are some examples:

Now that you know what an experimental variable is, remember just how important it is to control variables to ensure a fair test. And the next time someone makes a boisterous claim about a product or the cause of a phenomenon, ask yourself; “Has this been tested FAIRLY?”