🔎 Ask yourself
What did the researcher decide to change? Each different version of that thing is a level of the IV.
Experimental Method » Experimental Conditions and Control Groups
What you'll learn this session
Study time: 30 minutes
Cambridge spec: 7.1.14, 7.1.15, 7.1.16, 7.1.17, 7.1.18
- What experimental conditions are and how to spot them in a new study
- What an experimental group is
- What a control group is and why it is used as a comparison
- How to name the groups and conditions in a scenario you have never seen
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Unlock This CourseExperimental conditions
In Independent and Dependent Variables you learned that the independent variable (IV) is the thing the researcher changes, and that it has different levels. This lesson gives those levels a job title. The levels the researcher sets up on purpose are the experimental conditions. A version where the participants get none of the IV, such as no gum, is what a control group gets (more on that below).
Key terms:
- Experimental conditions: the levels of the independent variable that are manipulated in an experiment.
- Experimental group: the participants who experience the experimental condition(s).
- Control group: the participants who do not experience the experimental condition(s). They are used as a comparison group.
Learn this
Experimental conditions are the levels of the IV that the researcher manipulates. To find them, ask: what did the researcher change, and what were the different versions?
Spotting the conditions in a scenario
Worked example
A researcher wants to know whether chewing gum changes how many words pupils can remember. Pupils are shown a list of 20 words for two minutes. Some pupils chew gum while they study the list. Others study it without gum. Five minutes later, every pupil writes down as many words as they can.
IV: chewing gum while studying. Experimental condition: chewing gum while studying the list. The other version, no gum, is the second level of the IV. DV: the number of words recalled.
An experiment can have more than two levels. Imagine the researcher added a third version, where pupils chew mint gum. Now the IV has three levels: no gum, ordinary gum and mint gum. Ordinary gum and mint gum are two experimental conditions, and no gum is what the control group gets.
📝 Do not pick the DV
The thing measured at the end, such as words recalled, is the DV. It is never a condition.
Experimental groups
A condition is the version of the IV. A group is the people. The experimental group is the set of participants who experience the experimental condition.
In the gum study, the pupils who chew gum while studying are the experimental group. They get the treatment that the researcher wants to test.
Condition or group?
Condition = what happens (chewing gum). Group = who it happens to (the pupils who chew gum). Exam questions often ask for one or the other, so read the command word carefully.
Control groups
Now think about the gum study again. Suppose the pupils who chewed gum recalled 12 words on average. Is that good? You cannot tell. Perhaps pupils would have recalled 12 words anyway.
The researcher needs something to compare with. The control group is the set of participants who do not experience the experimental condition. In the gum study, the pupils who study the list without gum are the control group. They do the same task and the same recall test, but they do not get the IV.
🧪 Experimental group
Chews gum while studying. Recalls 12 words on average.
⚖ Control group
Studies without gum. Recalls 9 words on average.
Now the researcher can see an effect. The two groups were treated the same except for the IV, so the 3-word difference may be down to the gum. Without the control group, there would be nothing to compare the 12 with.
Why a control group matters
A control group lets the researcher see what happens without the IV. Comparing the two groups shows the effect of the IV on the DV.
Not every experiment needs a control group
Sometimes the researcher compares two or more different versions of the IV instead. For example, pupils might revise for 10 minutes or for 30 minutes before a test. Nobody gets "no revision", so both groups experience an experimental condition. Both are experimental groups, and there is no control group. Always check whether any group really misses out on the IV.
Applying it to a new scenario
Worked example
A researcher wants to know whether giving sticker rewards makes young children tidy up more. Thirty children are in a nursery. Fifteen children are told they will get a sticker for each toy they put away. The other fifteen children are asked to tidy up but are told nothing about stickers. The researcher counts the toys put away by each child.
- IV: whether a sticker reward is offered.
- Experimental condition: being offered a sticker for each toy put away.
- Experimental group: the fifteen children offered stickers.
- Control group: the fifteen children not offered stickers. They show how much tidying happens without the IV.
- DV: the number of toys put away.
Worked example: no control group
A researcher wants to know whether a short talk or a long talk about healthy eating changes pupils' lunch choices. One class hears a 5-minute talk. Another class hears a 20-minute talk. The researcher counts the fruit portions chosen at lunch.
The IV is the length of the talk, with two levels. Both classes hear a talk, so both experience an experimental condition. Both are experimental groups. There is no control group, because no class hears nothing.
Common mistakes
- Calling the control group "the group that does nothing at all". It does the same task and tests as the other group. It just misses the IV.
- Mixing up the group and the condition. The condition is what happens. The group is the people.
- Saying the control group is the "better" group. It is simply the comparison group.
- Thinking a control group is the same as a control. A control group is a set of participants. Controlling other variables is a different idea, taught in the next lesson.
- Naming a control group in a study where every group gets some level of the IV.
Exam-style question
A researcher investigates whether listening to a short story at bedtime helps young children to fall asleep faster. Forty children take part. Twenty children are read a story at bedtime. The other twenty children are put to bed with no story. A parent records the number of minutes each child takes to fall asleep.
(a) Identify the experimental condition in this study. [1 mark]
(b) Identify the control group. [1 mark]
(c) Explain why the researcher used a control group in this study. [2 marks]
Model answer
(a) Being read a story at bedtime (1).
(b) The twenty children who were put to bed with no story (1).
(c) The control group shows how long children take to fall asleep without a story (1), so the researcher can compare it with the story group and see the effect of the story on the time taken (1).
Exam tip
For part (c), do not just write "to compare". Say what is being compared: the time taken to fall asleep with the story and without it.