Independent, Dependent & Control Variables (Cambridge (CIE) IGCSE Biology): Revision Note

Independent, dependent & control variables

• A variable is anything that can change in an investigation

  • Most investigations test how one variable affects another

Independent variable

• The independent variable is the variable you change in the investigation

• Changing the independent variable may cause a change in the dependent variable

  • It is usually named in the aim, e.g. "the effect of temperature on…"

Dependent variable

• The dependent variable is the variable you measure or observe

  • It may change when you change the independent variable, i.e. the dependent variable depends on the independent variable

  • It is what you record in your results table, e.g. the number of bubbles per minute

Control variables

• Control variables are all the variables, other than the independent variable, that could affect the dependent variable

  • They are kept constant, so that only the independent variable affects the results

• A variable cannot be the independent variable and a control variable at the same time

• Examiner reports show that students lose marks by:

  • giving the dependent variable when asked for the independent variable

  • giving a control variable, such as time, as the independent variable

  • stating the same variable as both a control variable and the independent variable

Experimental range & values

• The experimental range is the lowest and highest values of the independent variable, e.g. from 20 °C to 60 °C

  • If the independent variable is continuous, e.g. temperature or concentration, use a range of values, e.g. five, spread evenly across the range

    • Choose a range that is:

      • wide enough to show a clear pattern, e.g. that includes temperatures above and below the optimum for an enzyme

      • safe

      • practical

  • If the independent variable is made up of categories, e.g. resting or exercising, use each category and keep every other variable the same

• Repeat each value, and calculate a mean; this reduces the effect of random errors on the results

• When you describe how you will change the independent variable, give the actual values, e.g. for sugar concentration: 0.5%, 1.0%, 1.5%, 2.0% and 2.5%

• Examiner reports show that students lose marks by naming the independent variable without saying how it will be changed

How & why variables are controlled

• When describing control variables, you need to:

  • describe how a variable is kept constant

    • Say exactly what is kept the same, e.g. "the volume of amylase", not "the amount of amylase"

    • Give values where you can, e.g. "5 cm³ of 1% amylase solution"

    • Say how you keep it constant, e.g. "use a water bath at 30 °C" or "keep the lamp 20 cm from the pond weed"

  • explain why

    • Controlling variables other than the independent variable ensures that nothing else affects the results

    • Say how the variable would affect the dependent variable if it changed

• Examiner reports show that students lose marks by:

  • using vague words such as "amount" when describing a control variable

    • Always refer to, e.g. volume, mass or concentration

  • not giving enough detail about how a variable is kept constant

What is a control experiment?

• A control experiment (also known as an experimental control) is a separate setup run alongside the main investigation; it is set up in exactly the same way as the main test, but with the specific factor being tested removed or inactivated.

• The purpose of a control experiment is to:

demonstrate that any observed change in results is caused only by the factor being tested, and not by another variable

• Examples include:

  • the same volume and concentration of boiled (denatured) enzyme instead of active enzyme, in an enzyme investigation

  • an equal volume of water instead of the test solution

  • an equal volume of glass beads instead of germinating seeds in a respiration investigation

• A control experiment is not the same as a control variable

  • Control variables are kept the same in every test, but a control experiment is a separate set-up used for comparison