How experiments show cause
Random assignment, a control group and blinding are what let an experiment show that one thing causes another. Learn to check for each before trusting a claim.
PsychologyRead claims about the mindAbout 5 min: 3 reading, 2 practiceUpdated
Educational, not medical or mental-health advice.
- Name the parts of an experiment (independent and dependent variable, control group, random assignment)
- Explain placebos and blinding and why they matter
- Judge whether a study's design can support a causal claim
Builds toward: Read claims about the mind. Judge a headline or a study about psychology before believing it or passing it on.
Read claims about the mind: lesson 2 of 5
- Read claims about the mind
- Linked is not the same as caused 6 min
- How experiments show cause 5 min
- Samples, effect sizes and "significant" 5 min
- What holds up: the replication crisis 7 min
- Popular psychology myths, checked 7 min
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Pairs with OpenStax, Psychology 2e, section 2.3 "Analyzing Findings"
What it is
An experiment changes one thing on purpose and measures what happens, while keeping everything else the same. That is what lets it show cause, not just a link.
- The independent variable is what the researcher changes (for example, whether people drink coffee).
- The dependent variable is what they measure (for example, a memory test score).
- The experimental group gets the change; the control group doesn't, and is compared with it.
- Random assignment means chance, such as a coin toss or a computer, decides who goes in which group. Then the groups should be alike on average in everything else: age, sleep, mood, ability. Any difference afterwards is likely down to the change.
Quick check
Tap an answer to check it.
A researcher lets volunteers choose whether to join the "coffee" group or the "no coffee" group. What is the problem?
- Choosing their own group is exactly what makes the groups unequal
Show answer
The groups may differ in other ways, such as sleepiness, so a difference can't be pinned on the coffee. Random assignment is what makes the groups alike in everything except the change being tested.
Why it matters
Placebos. If people expect to feel better, many do, even from a pill with nothing in it. This is the placebo effect. So a fair test gives the control group a placebo: something that looks the same but lacks the active part. Then the only difference between the groups is the thing being tested.
Blinding. In a single-blind study, participants don't know which group they are in. In a double-blind study, the researchers who deal with them and score the results don't know either, so their hopes can't nudge the results, even without meaning to. The double-blind, placebo-controlled, randomised trial is the standard for testing treatments.
What an experiment can't do. Many questions can't be tested this way for ethical or practical reasons: you can't randomly assign people to smoke, to be poor, or to have a difficult childhood. For those, researchers combine many kinds of evidence, which is slower and less certain. And a well-run experiment still only shows what happened to these people, in these conditions.
Worked example
Claim: "A 10-minute brain-training game improves memory." Design a fair test.
- Independent variable: playing the game or not. Dependent variable: score on a memory test.
- Control group: plays a different 10-minute game with no memory training, so both groups spend the same time on screens and think they're doing something useful.
- Random assignment: a computer splits 200 volunteers into the two groups.
- Blinding: the people scoring the memory tests don't know who played which game.
- Measure the right thing: test memory on tasks unlike the game. Getting better at the game itself isn't the same as better memory in daily life.
If the training group still does better, the game probably caused it, for people like these volunteers.
Common mistake
Calling any study an "experiment". Many headlines describe surveys or observational studies, where researchers only watch and measure. They can find links but not prove cause. Ask: did the researchers decide who got what, by chance? If not, it isn't an experiment in this sense.
Key terms
- experiment
- a study that changes one thing on purpose and measures the effect
- independent variable
- what the researcher changes
- dependent variable
- what the researcher measures
- control group
- the group that doesn't get the change, used for comparison
- random assignment
- using chance to decide who goes in each group
- placebo
- a fake treatment that looks like the real one
- double-blind
- neither the participants nor the researchers who run and score the study know who is in which group
- observational study
- a study that measures what happens without changing anything
Practise the key terms
Tap a card to turn it over. 8 cards.
| Front | Back |
|---|---|
| experiment | a study that changes one thing on purpose and measures the effect |
| independent variable | what the researcher changes |
| dependent variable | what the researcher measures |
| control group | the group that doesn't get the change, used for comparison |
| random assignment | using chance to decide who goes in each group |
| placebo | a fake treatment that looks like the real one |
| double-blind | neither the participants nor the researchers who run and score the study know who is in which group |
| observational study | a study that measures what happens without changing anything |
Practice
Quick quiz
Tap an answer to check it.
In a study of whether music helps people concentrate, what is the dependent variable?
- That is what the researcher changes: the independent variable
Show answer
The concentration test score. The dependent variable is what gets measured.
Why give the control group a placebo pill instead of nothing?
- The reason is fairness, not cost
Show answer
So that expecting to feel better is the same in both groups. Then any extra effect in the drug group comes from the drug, not from expectation.
What makes a study double-blind?
- Blinding is about who knows the group, not repetition
Show answer
Neither the participants nor the researchers running and scoring it know who is in which group. Double-blinding stops both participants' and researchers' expectations from shaping the result.
A study found that children who play a musical instrument do better at maths. Can it show music causes better maths?
- A clear difference is still only a link without random assignment
Show answer
Not if the children chose their own activities; it's observational, and other factors could explain the link. Families who pay for music lessons may differ in many ways that affect maths.
Build the sentence
Put the steps of a fair experiment in order.
Show answer
Decide what you'll change and what you'll measure Randomly assign people to groups Give one group the change and the other a placebo Measure the results without knowing who is in which group Compare the groups. Random assignment before the change, and blind measurement after it, are what make the comparison fair.
In short
- An experiment changes one thing (independent variable) and measures another (dependent variable).
- Random assignment makes the groups alike, so a difference can be put down to the change.
- A placebo makes expectations equal; double-blinding stops anyone's hopes shaping the results.
- Surveys and observational studies show links, not cause.
- Some questions can't be tested by experiment; then the evidence is less certain.
Mastery check
Answer without looking back. Score 80% or more and the lesson is marked done. Your result is kept only in this browser.
Check yourself
Answer in your head first, then open the answer. Nothing is saved.
Why is random assignment so important?
Show answer
It makes the groups alike on average in everything except the change being tested, so a difference afterwards can be put down to that change.
What is the placebo effect?
Show answer
Improvement that comes from expecting a treatment to work, even when it has no active ingredient.
A news story says "a study proves sleep makes students happier" and describes a survey. What would you point out?
Show answer
A survey can show that sleep and happiness are linked, but not that one causes the other; happier students might sleep better, or a third factor might affect both.
Further reading
- OpenStax, Psychology 2e, section 2.3, free online.
- Evans, I., Thornton, H., Chalmers, I., & Glasziou, P. (2011). Testing Treatments: Better Research for Better Healthcare (2nd ed.). Pinter & Martin. Free online at testingtreatments.org.
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