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Biology · Chapter 2: The Scientific Method (Solving a Biological Problem)
Experiments, Variables and Controls
How to plan a fair test: independent, dependent and controlled variables, experimental and control groups, and reliable results.
Key termRememberExampleWatch out
01Concept
An experiment is a carefully planned test of a hypothesis, carried out under controlled conditions. A good experiment changes only one thing at a time, so that we know exactly what caused the result.
Variables
A variable is any factor that can change in an experiment. There are three kinds:
| Variable | Meaning | In the fertiliser experiment |
|---|---|---|
| Independent variable | The one factor the scientist changes on purpose | Amount of fertiliser |
| Dependent variable | The factor that changes as a result and is measured | Height of the wheat plants |
| Controlled variables | All the other factors, kept the same for every group | Type of seed, soil, pot size, water, light, temperature |
Experimental group and control group
- The experimental group receives the treatment (the independent variable is changed). Plants given fertiliser.
- The control group does not receive the treatment. Everything else is the same. Plants given no fertiliser.
The control group is used for comparison. Without it, we could not tell whether the plants grew because of the fertiliser or because of good weather that month.
Making results reliable
- Use many individuals, not one. One plant may be weak by chance; 20 plants give a fair picture.
- Repeat the experiment and take the mean (average) of the results.
- Measure carefully with suitable instruments, and record readings at once.
- Change only one variable at a time.
02Virtual Lab
Try this topic for yourself in the virtual lab. It opens on its own page with steps, tasks and questions.
03Real World Examples
Seed trials
When a company says its cotton seed gives more yield, agricultural officers grow it beside an ordinary variety (the control) in the same field, with the same water and fertiliser, before recommending it.
Medicine trials
In a medicine trial, the control group gets a placebo (a tablet with no medicine). Neither the patients nor the doctors know who got which, so that their expectations cannot affect the result.
Kitchen experiment
To find out whether lemon juice stops a cut apple from turning brown, put lemon juice on one half and nothing on the other half (the control). Leave both in the same place for the same time.
04Practice Questions
Try to answer in your own words first, then open the model answer.
Short questions
Q1.Define an experiment.Show answer
Answer
An experiment is a planned test of a hypothesis carried out under controlled conditions.
Q2.What are independent, dependent and controlled variables?Show answer
Answer
- Independent variable: the factor the scientist changes.
- Dependent variable: the factor that changes as a result and is measured.
- Controlled variables: the factors kept the same in all groups.
Q3.What is a control group? Why is it necessary?Show answer
Answer
A control group is the group that does not receive the treatment but is otherwise treated exactly the same. It is needed for comparison, to show that the result was caused by the independent variable and nothing else.
Q4.A student tests whether temperature affects how fast yeast makes gas. Name the independent, dependent and two controlled variables.Show answer
Answer
- Independent: temperature of the water bath.
- Dependent: volume of gas (or the size of the balloon) after 20 minutes.
- Controlled: amount of yeast, amount of sugar, volume of water, time.
Q5.Why should an experiment use many plants and be repeated?Show answer
Answer
Because one individual may give an unusual result by chance. Using many individuals and repeating the experiment, then taking the mean, makes the results reliable.
Long questions
Q6.Design an experiment to test the hypothesis: "Fertiliser increases the growth of wheat seedlings." Mention the variables and the control.Show answer
Answer
- Take 20 wheat seedlings of the same age and height, in 20 identical pots with the same soil.
- Experimental group: 10 pots get fertiliser. Control group: 10 pots get no fertiliser.
- Independent variable: fertiliser. Dependent variable: height of the seedlings. Controlled variables: soil, pot size, water, light, temperature.
- Measure the height of every seedling every 3 days for 3 weeks and record it in a table.
- Work out the mean height of each group and draw a line graph.
- If the fertilised group grows clearly taller, the hypothesis is supported. Repeat the experiment to check.