How do psychologists design experiments and control variables?
Experiments and variables: independent and dependent variables, the hypothesis, experimental designs, extraneous variables, and laboratory, field and natural experiments.
A focused answer to AQA GCSE Psychology 3.4, covering independent and dependent variables, hypotheses, experimental designs, extraneous variables, and laboratory, field and natural experiments.
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What this dot point is asking
AQA wants you to define independent and dependent variables, write and operationalise hypotheses, describe experimental designs and extraneous variables, and compare laboratory, field and natural experiments. Research methods runs through both papers and is applied to scenarios, so you must be able to spot variables and judge designs in an unfamiliar study.
Variables and hypotheses
A hypothesis is a precise, testable prediction. An alternative (experimental) hypothesis predicts a difference (for example, "participants recall more words after sleep than after no sleep"), while a null hypothesis predicts no difference. A good hypothesis is operationalised so it states exactly what will be measured.
Experimental designs
Extraneous variables and types of experiment
An extraneous variable is any variable other than the IV that could affect the DV; if it is not controlled it can become a confounding variable that ruins the conclusion. Researchers control extraneous variables through standardised procedures. The types of experiment differ in control and realism: a laboratory experiment is run in a controlled setting (high control, lower ecological validity); a field experiment is run in a real-life setting (more realistic, harder to control); and a natural experiment studies an IV that varies naturally (the researcher does not manipulate it, useful when manipulation would be unethical or impossible).
Try this
Q1. Define the dependent variable. [1 mark]
- Cue. The variable that is measured in an experiment.
Q2. Identify one advantage of a repeated measures design. [1 mark]
- Cue. It controls for individual differences (the same people do every condition).
Q3. Explain one strength of a field experiment over a laboratory experiment. [2 marks]
- Cue. It takes place in a real setting, so behaviour is more natural (higher ecological validity).
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20184 marksA researcher tests whether background music affects how many maths questions students answer correctly. Identify the independent variable and the dependent variable, and write a suitable hypothesis. (Paper 1, Section D)Show worked answer →
An applied 4-mark item that rewards correct identification of each variable plus a properly operationalised hypothesis.
The independent variable (IV) is what the researcher manipulates: the presence or absence of background music (music versus no music). The dependent variable (DV) is what is measured: the number of maths questions answered correctly. A suitable hypothesis predicts the effect and is operationalised, for example: "Students will answer more maths questions correctly when working in silence than when working with background music." (A null hypothesis, predicting no difference, is also creditworthy if asked for.)
Markers reward correctly naming the IV (music or not) and DV (number correct) and a clear, testable hypothesis that states the predicted difference. A vague hypothesis ("music affects maths") without an operationalised direction loses marks.
AQA 20214 marksExplain one strength and one weakness of conducting a laboratory experiment. (Paper 1, Section D)Show worked answer →
A 4-mark item that rewards one developed strength and one developed weakness.
Strength: a laboratory experiment is highly controlled, so extraneous variables can be reduced and the researcher can be more confident that the IV caused the change in the DV (high internal validity), and the controlled procedure makes it easy to replicate. Weakness: the artificial setting can lower ecological validity, because behaviour in a lab may not reflect how people act in everyday life, and participants may behave differently because they know they are being studied (demand characteristics).
Markers reward the control and replicability strength and the artificiality or demand-characteristics weakness, each explained rather than just stated.
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Sources & how we know this
- AQA GCSE Psychology (8182) specification — AQA (2017)