Planning an investigation: variables, controls, range and repeatsSpec S1
In short
Planning an investigation means stating exactly what you change, what you measure, what you keep the same and how. The independent variable is changed, usually over at least five evenly spaced values; the dependent variable is measured with named apparatus. Keep controlled variables constant, include a control experiment, repeat each test at least three times and calculate a mean.
Paper 5 and Paper 6 often ask you to plan an investigation, sometimes one you have never done. A good plan says exactly what you change, what you measure, what you keep the same and how.
- Independent variable
- The variable that is changed in a scientific experiment by the scientist. Changing an independent variable may cause a change in the dependent variable.
- Dependent variable
- The variable that is observed or measured in a scientific experiment. It may change based on changes made to the independent variable.
- Controlled (constant) variables
- Other variables that could affect the dependent variable, so they are kept the same in every test.
- Control experiment
- A set-up that is identical except that the factor being tested is missing or inactive, for example boiled enzyme or no plant. It shows that any change is caused by the factor being tested.
What a full plan includes
- The independent variable and a suitable number and range of values: usually at least five values, evenly spaced, such as 20, 30, 40, 50 and 60 °C.
- The dependent variable and exactly how you measure it, with the apparatus: for example time with a stop-clock, volume with a measuring cylinder or syringe, mass with a balance.
- The variables you keep constant, how you keep each one constant and why it matters.
- A suitable control experiment.
- Repeats: do each test at least three times and calculate a mean.
- The risks and safety precautions.
- How you will record the results (a table) and process them (a mean, a rate, a graph) to reach a conclusion.
| Part of the plan | Example |
|---|---|
| Independent variable | temperature: 20, 30, 40, 50 and 60 °C, using water baths |
| Dependent variable | time taken for the starch to disappear, found by testing a drop of the mixture with iodine solution every 30 s until it stays orange-brown |
| Constant: volume and concentration of starch and amylase | measure each with a syringe or measuring cylinder, because more enzyme or substrate changes the rate |
| Constant: pH | add the same volume of a buffer solution, because pH also affects enzyme activity |
| Control | boiled (denatured) amylase with starch: the iodine stays blue-black |
| Repeats | three tests at each temperature, then a mean |
Leave the enzyme and the substrate in separate tubes in the water bath for a few minutes before mixing, so both reach the test temperature.
Predictions and safety
A reasoned prediction gives the expected result and a reason, for example: 'the time will decrease from 20 °C to about 40 °C because the molecules have more kinetic energy and collide more often, then increase above 40 °C because the enzyme is denatured.'
- Wear eye protection when using chemicals, indicators or hot water.
- Take care with hot water baths and hot test-tubes: use a test-tube holder.
- Cut away from your body on a white tile when using a scalpel.
- Keep ethanol away from flames because it is flammable.
- Wash your hands after handling soil, plants or living organisms.
For a safety mark, link the precaution to the hazard, for example 'wear eye protection because iodine solution is an irritant'. 'Be careful' scores nothing.
Written and checked against the Cambridge IGCSE Combined Science (0653) specification · Updated October 2026