A control is a comparison set-up identical to the test except that the factor you are investigating is absent or unchanged. Exam questions give you a method and ask what the control should be, or why one is needed.
This skill opens Biology data and investigations because every later skill, from sample size to validity, assumes you can see what is being compared.
What is the control actually doing?
Suppose a result changes. You want to say the factor caused it. Without a control, something else could have caused the change, such as time passing, the container or the room.
The control answers one question: what would have happened without the factor? Then any difference between test and control can be linked to the factor.
How to choose a control, step by step
- Name the factor being tested (the independent variable).
- Name what is measured (the dependent variable).
- Write the control set-up: everything as in the test, but with the factor absent, or at its normal level.
- List the control variables: the conditions that must be identical in every set-up.
- State what the control shows in one phrase: “to show the change is due to X and not to Y”.
Worked example
All details are invented for practice. A student wants to know whether an extract made from crushed garlic slows the growth of mould on bread. She puts 2 cm³ of garlic extract on a slice of bread in a sealed dish and records the mould area after 5 days.
Step 1, factor tested: garlic extract.
Step 2, measured: area of mould on the bread, in cm².
Step 3, control: a slice of the same bread in an identical dish, given 2 cm³ of water in place of the extract. Water matches the liquid volume but lacks the extract.
Step 4, control variables: same type and size of bread, same volume of liquid, same dish, same temperature, same place and the same 5 days.
Step 5, what it shows: any difference in mould area is due to the extract and not to wetting the bread.
Notice that the test and the control differ in one thing only.
The mistake to watch for
A common answer to “suggest a control” is to describe another test.
Mistaken answer: “Use a different food, such as cheese, with the same extract.”
This changes the food, so it is a different experiment. It does not show what happens without the extract.
The correction is to keep everything the same and change only the tested factor to absent. Another slip is listing control variables when the question asks for a control. Read the command word and answer the one asked.
Check yourself
All experiments are invented.
1. A student tests whether light affects the germination of lettuce seeds by putting seeds in a dark cupboard. What should the control be?
Show answer
A set of the same number of the same type of lettuce seeds placed in light, with the same water, temperature and time. The only difference is light, so any change in germination is linked to light.
2. In an experiment on the effect of a detergent on amylase, a tube has amylase, starch and detergent. Suggest a control and say what it shows.
Show answer
A tube with the same amylase, the same starch and the same volume of water instead of detergent, at the same temperature. It shows whether starch breakdown happens without the detergent, so any difference is due to the detergent.
3. A question asks for two control variables in an experiment on the effect of temperature on yeast respiration. Give two.
Show answer
Any two of: the mass (or volume) of yeast, the volume and concentration of glucose solution, the time the gas is collected, the same apparatus. Each must be the same at every temperature.
Where this leads next
Once you can name a control, identify a sample-size limitation to judge whether the comparison is strong enough. The inheritance model board shows how a model ratio differs from a small real sample.
If you know the idea yet still lose the mark because of wording, that is the kind of pattern our teachers look for in online one-to-one Biology tuition.