A lab experiment is defined by control
A laboratory experiment manipulates the IV and measures the DV while controlling extraneous variables in a highly controlled setting. The defining feature is control, not the location. (IV = what the researcher changes; DV = what is measured; extraneous variable = anything else affecting the DV.) Because control removes rival explanations, it can test cause and effect: if only the IV changes and the DV moves with it, the IV is the cause.
Strengths: control, cause-effect, replication
Learn the strengths as a chain. High control — all but the IV is held constant, removing rival explanations for a change in the DV. Cause and effect — only the IV changes, so a change in the DV is attributed to it, giving high internal validity. Standardised procedures let the study be replicated, supporting reliability, and the DV is measured precisely. In short: high control lets it show cause and effect and be replicated.
Weaknesses shape what it suits
Control has a cost; the weaknesses show what the method does not suit. Low ecological validity — a controlled, artificial task (e.g. memorising unrelated words) may not reflect everyday life, so findings may not generalise. Demand characteristics — participants know they are studied and may change how they behave. Suitability: it fits a clear causal test of an isolable variable (noise on memory), not rich real-world behaviour, which control would destroy.
Drawn from real examiner reports.
'Done in a lab' is not a definition
Asked what a laboratory experiment is, many answer "an experiment done in a laboratory". That just repeats the term — a tautology — earning no credit. A good definition names the features: the researcher manipulates the IV and measures the DV, extraneous variables are controlled, so cause and effect can be tested. Give the defining features, not the name restated.
"Conducted in a laboratory" is a tautology, not a definition of a laboratory experiment (Nov 2020 P2 Q03a).
Explain a strength — say why it matters
Explain a strength/weakness is not describe one. Naming a feature earns little; the marks come from why it is a strength. Weak: "a strength is high control." Full: "all but the IV is held constant, so a change in the DV can only be due to the IV — cause and effect." Give the feature, why it follows, and the consequence — the consequence carries the mark.
Explain-a-strength/weakness answers must not merely describe (June 2023/2024 P2 Summaries); identification without justification (June 2024 P1 Q11b, Q15b).
On a scenario, generic evaluation scores 0
When you must evaluate the laboratory experiment used in a study, an evaluation floating free of it earns no marks: AO3 needs AO2. Not application: naming the researcher or repeating the stem. Application: use the study's detail — "the list, time and room were the same, so only the noise differed". Write the applied point first.
AO3 cannot be awarded without AO2 (June 2023 P2 Q31); purely generic responses to scenario questions cannot be credited (June 2019 P1 Summary).
The room doesn't make it a lab experiment
Students think the label depends on the room — a lab study counts, elsewhere does not. That is the wrong test. What makes it a laboratory experiment is high control: the IV is manipulated and extraneous variables held constant. A controlled room that is not literally a lab can still qualify. Ask not "was it in a lab?" but "was the IV manipulated and variables controlled?"
High control isn't high in every validity
A tightly controlled lab experiment is assumed strong in every way, but control buys one validity while costing another. Internal validity is high — controlled variables mean a DV change is due to the IV (cause and effect). Ecological validity can be low — the artificial setting may not represent real life. So it is strong on internal but weak on ecological validity.
Lab, field and natural experiments differ
A study is misclassified by fixing on one feature. Not a field experiment just because a variable was manipulated — a field experiment manipulates it in a real-life setting, a lab in a controlled one. Nor a natural experiment because the topic sounds everyday — there the IV is a naturally occurring difference. Test: was the IV manipulated in a controlled setting?
Develop AO1 in the methods essay
In the 12-mark methods essay, AO1 is the persistent weakness: candidates make bare statements like "it is controlled" and develop little else, capping the mark. Top-band AO1 explains the sub-features — IV manipulated, DV measured, extraneous variables controlled, procedures standardised — saying what each means and why. Don't list features; explain them.
AO1 is the persistent weakness in the methods essay — simple statements developed little further (June 2022 P2 Q5; June 2023 P2 Q5).
The 12-mark essay: AO1, AO2, AO3, conclude
On the 12-mark Evaluate essay: develop AO1 (explain features, not bare statements); use the stimulus study constantly for AO2; balance AO3 across several issues — internal vs ecological validity, demand characteristics — then conclude. Starting with AO3 leaves AO2 thin.
Build a strength: feature, why, consequence
For "explain a strength/weakness", write three parts: the feature ("high control"), why it follows ("only the IV differs"), and the consequence ("the DV change is down to the IV — cause and effect"). The consequence carries the mark, so never stop at the feature.
Write the applied point before you judge
On a scenario, write the applied (AO2) point first, then evaluate. Anchor to the study's detail — IV, DV, what it held constant, the setting — before judging. AO2 must be present, since AO3 depends on it; a general strength that never touches this study scores nothing.
A laboratory experiment is a research method in which the researcher manipulates the independent variable and measures the dependent variable while controlling extraneous variables in a highly controlled setting. The defining feature is control, not the room.
Laboratory experiment — a controlled experiment in which the IV is manipulated, the DV is measured, and extraneous variables are held constant, so that cause and effect can be tested.
Independent variable (IV) — the variable the researcher deliberately changes.
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