In one line: An experiment changes one thing on purpose and measures what happens. It is the one method that can show cause and effect.
You have already run an experiment.
Your phone battery keeps dying by lunchtime. So for one day you turn the screen brightness right down and change nothing else: same apps, same hours of use.
The battery lasts until the evening. Brightness was the only thing you changed, so you can say it caused the difference.
The study on this page: Psychologists test behaviour the same way. Take a question many students ask: does music help or hurt memory?
A researcher has one group learn 20 words with music playing and another group learn them in silence, then counts how many words each person remembers.
Change one thing
the independent variable
- Decided by the researcher
- Music or silence
Keep the rest the same
the controls
- Same word list
- Same time to learn it
Measure the result
the dependent variable
- Words remembered out of 20
The thing the researcher changes on purpose is the independent variable (IV): music or silence.
The thing the researcher measures is the dependent variable (DV): how many words people remember.
If the IV is the only difference between the groups, a difference in the DV can be put down to the IV. Showing that one thing causes another is called causality.
Remember: Change one thing. Measure another. Keep the rest the same.
Free preview
This is the free notes preview
You're reading the free notes. Aimnova Pro unlocks the full study experience — and you can try it with your first topic free to keep:
- FlashcardsLock in vocabulary and key terms with spaced repetition.
- Practice questionsAnswer exam-style questions and get instant AI marking.
- Mock exams & past-paper vaultSit full mocks and see exactly how examiners award marks.
- Personalised study planA daily plan built around your exam date and weak areas.
Key idea: Every experiment is built from the same five parts. Learn them once and you can describe any experiment.
Stay with the music study. Forty students learn a list of 20 words, some with pop music playing and some in silence.
The five parts
IV and DV, made measurable
Each variable is stated so exactly that it can be measured. This is called operationalising it.
Here: the IV is pop music or silence. The DV is not "memory" but "words written down correctly out of 20".
Hypothesis
A prediction that can be tested: what the IV will do to the DV.
Here: students who learn in silence will remember more words than students who learn with music.
Experimental and control group
The experimental group gets the treatment. The control group does not, so there is something to compare with.
Here: the music group is the experimental group and the silence group is the control group.
Random allocation
Chance decides who goes into which group, for example by drawing names from a bag.
Strong and weak memories end up spread across both groups, so the groups start out alike.
Controls
Anything else that could change the DV is an extraneous variable: noise, time of day, how hard the words are.
Keeping these the same for everyone is called using controls. Same list, same instructions, same timing: a standardised procedure.
Variables · Hypothesis · Groups · Random allocation · Controls
When a control fails: Suppose the music group is tested at 8 am and the silence group at noon.
Time of day now changes together with the IV. It has become a confounding variable, and nobody can tell whether the music or the early start lowered the scores.
Learn what examiners really want
See exactly what to write to score full marks. Our AI shows you model answers and the key phrases examiners look for.
Key idea: Before an experiment starts, the researcher decides who does which version of the task. This choice is called the experimental design.
The music study has two versions of the task: learning with music and learning in silence. Each version is called a condition.
There are only three ways to share out the conditions.
1. Everyone does both. The same 40 students learn one list with music and another list in silence.
2. One each. 20 students learn with music. 20 different students learn in silence.
3. Pairs. Each student is paired with someone who has a similar memory score. One of the pair learns with music, the other in silence.
Repeated measures
- Everyone does both conditions
- Strength: each person is compared with themselves
- Strength: fewer people are needed
- Limit: order effects, and the aim is easier to guess
Independent measures
- Each person does one condition
- Also called independent samples
- Strength: no order effects, and the aim is harder to guess
- Limit: the groups may differ from the start
Matched pairs
- Similar people are paired, then split
- Strength: the groups are alike, with no order effects
- Limit: slow, and no match is perfect
Remember: Both = repeated measures. One each = independent measures. Pairs = matched pairs.
Both: watch the order. One each: check the groups are alike. Pairs: slow to set up.
Order effects are the big risk in repeated measures.
The fix is counterbalancing: half the students do music first and half do silence first.
How to spot the design: Ask one question: did each student learn with music and also in silence?
Yes: it is repeated measures.
No: were students put in pairs first? Paired: matched pairs. Not paired: independent measures.
Key idea: Two questions sort every experiment. Did the researcher set the IV and randomly allocate people? And where did it happen?
The four types
True experiment (laboratory)
The researcher controls everything: who is in each group, and the room they work in.
Music study: 40 students come to a quiet, empty classroom set up for the test (the laboratory). Names drawn from a bag decide who learns the words with music and who learns them in silence.
Real study: Loftus and Palmer (1974) showed students a film of a car crash. Each group was asked the same question with a different verb: how fast were the cars going when they "smashed", or "hit", each other? "Smashed" gave the highest speed estimates.
Good: the room, the words and the time are the same for both groups. So if the music group remembers fewer words, it must be because of the music. That is what "showing cause and effect" means.
Bad: a quiet test room is not like real life.
Field experiment
The same as a true experiment, but in a real place instead of a test room.
Music study: in a normal French lesson, one class learns 20 new words with music playing and another class learns them in silence. Next lesson, both take their usual vocabulary quiz.
Real study: Levine and colleagues (2005) worked with Manchester United fans. As each fan walked between two university buildings, a runner fell in front of them and seemed hurt. Fans helped far more often when he wore a Manchester United shirt than a Liverpool or a plain one. The researchers changed one thing, the shirt, in a real place.
Good: it feels like an ordinary lesson, so students act as they normally do.
Bad: a classroom is harder to control (corridor noise, friends chatting), and the students did not know they were in a study.
Quasi-experiment
Groups made by something about the person.
Students are sorted by who they are or what they already do, such as their age or a habit. The researcher cannot change that, so she cannot draw names from a bag.
Music study: the researcher asks who usually revises with music on and who revises in silence. Each group learns the 20 words its usual way, then everyone takes the same word test.
Good: it studies students' real habits, and works when the groups cannot be chosen.
Bad: the groups may differ in other ways. Students who revise in silence might just be more careful learners, so a better score may not be because of the silence.
Natural experiment
Groups made by something that happened to them.
An event in real life, such as a new school rule, changes things for people. The researcher did not cause it. She only compares before and after, or people it happened to with people it did not.
Not the same as quasi: quasi sorts people by what they are like; natural sorts them by what happened to them.
Music study: from January, the head teacher decides to play music in every classroom. The researcher had no say in it. She compares the word test scores from December (no music) with those from March (music).
Real study: Maguire and colleagues (2000) scanned the brains of London taxi drivers and of men who did not drive taxis. A part used for remembering places, the back of the hippocampus, was bigger in the drivers, and bigger the longer they had driven. Nobody made them taxi drivers: years of learning London's streets happened in real life.
Good: it studies real changes nobody could set up on purpose.
Bad: the least control. Other things may have changed too, such as a new teacher in January.
True · Field · Quasi · Natural
Natural experiments can be run in two ways.
Longitudinal design
- Longitudinal design: the same class takes the word test in December, March and June
- It shows how the music changes them over time
Cross-sectional design
- Cross-sectional design: Year 7, Year 9 and Year 11 all take the word test on the same day
- The years are compared
Can you say the music caused it?: Only if chance decided the groups, like names drawn from a bag. Then the two groups start out alike, and the music is the only difference.
In a quasi or natural experiment: Chance did not decide. The groups may have been different from the start: the students who revise in silence might just be more careful learners.
So you can say the groups scored differently, but not for certain that the music caused it.
Never wonder what to study next
Get a personalized daily plan based on your exam date, progress, and weak areas. We'll tell you exactly what to review each day.
Control helps and hurts: Tight control in an experiment cuts both ways: it helps you find the cause, and it hurts how real the study feels.
It helps
- In the music study, everyone learns the same words in the same quiet room
- Only the music differs, so we can say the music caused the difference in scores
It hurts
- Nobody really revises a list of random words in a silent test room
- So the result may not match how students revise at home, with their own notes and their phone nearby
Strengths
- Shows cause: only the music differed between the groups
- Room, words and time were the same for all
- Another school can repeat it exactly
- A score out of 20 is easy to compare
Limits
- A quiet test room is not like revising at home
- Students may guess the aim and try harder in silence
- A score does not say why music made a difference
- Harmful things, like stress, cannot be tested on purpose
Three exam words name these points
- Internal validity: high in the music study, because only the music differed.
- Ecological validity: low, because a quiet test room is not a bedroom or a library.
- Mundane realism: low, because nobody revises by learning a random word list.
Exam tip: Tie every strength or limit to the study.
Not "it had high control", but "every student had the same list and the same two minutes, so only the music differed".
Ready to ace Experimental methods?
You've seen the preview. Here's what you'll unlock:
Bite-Sized Study Notes
Full syllabus broken into chunks that stick
Exam Practice
See exactly how to score full marks
AI Feedback
Know exactly what examiners want
Smart Study Plan
Know exactly what to study next