Reconstruct the intended comparison
List the question, independent variable, dependent variable and allocation process. A critique needs to connect a design feature to a conclusion rather than merely label a sample small or a study subjective.
Separate random sampling, which concerns selecting people from a population, from random assignment, which allocates sampled participants to conditions.
Worked example: does music improve recall?
Imagine 20 volunteers tested in silence in the morning and another 20 tested with music in the evening. Everyone studies the same word list and the outcome is the number recalled correctly. Music is the intended manipulated factor.
However, time of day changes with the condition. A difference cannot isolate the effect of music. Fatigue is a possible alternative explanation, not an established cause in these fictional data.
Turn criticism into a feasible improvement
Run both conditions at comparable times, randomly allocate participants and standardise instructions. Random assignment helps reduce systematic differences but does not guarantee perfect balance in a small sample.
If participants complete both conditions, consider order and practice effects. Explain which problem an improvement addresses rather than asserting that randomisation solves everything.
Self-check: match the conclusion to the evidence
Write three sentences: the observed contrast, the design feature limiting causal interpretation, and a change addressing it. The key is that music and time are confounded, so recall differences cannot be attributed to music alone.
Then ask who volunteered and where generalisation is justified. External validity is distinct from repairing the experimental comparison. Use public or fictional materials and independently author assessed work.
References
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