Question at position 30 Why do individual differences (e.g., age, baseline self-control) typically not threaten internal validity in a well-designed experiment, even if they influence the dependent variable?Because random assignment distributes individual differences evenly across conditions, making them unlikely to explain group differencesBecause individual differences are irrelevant once the dependent variable is measuredBecause individual differences increase external validity, not internal validityBecause individual differences only matter in correlational studies

Question

BlackTom AI · Accepted Answer

Random assignment ensures that participant characteristics (age, baseline self-control, temperament) are, on average, distributed equally across experimental conditions. This is the key mechanism that protects internal validity: it prevents those individual differences from systematically aligning with the independent variable and producing a spurious effect.

Relevant concepts:
- Internal validity: the degree to which observed effects can be attributed to the manipulated independent variable rather than confounds.
- Confound: a variable that varies systematically with the independent variable and could explain group differences on the dependent variable.
- Random assignment: allocating participants to conditions by chance so that nuisance variables are expected to be balanced across groups.

Step-by-step reasoning:
1. Identify the threat: individual differences can influence the dependent variable (DV) and so could be alternative explanations for group differences.
2. Apply random assignment: when participants are randomly assigned, each person has an equal chance of being in any condition. Therefore, characteristics like age or self-control are not expected to cluster in one condition.
3. Use probability/law of large numbers: with adequate sample size, random assignment makes the expected (mean) values of those characteristics similar across groups, removing systematic bias.
4. Conclusion: because these variables are distributed evenly, they are unlikely to cause the observed between-group differences; the manipulated variable remains the most plausible cause.

Why the correct option is right:
It states that random assignment distributes individual differences evenly, making them unlikely to explain group differences — exactly the mechanism that preserves internal validity.

Why other options are incorrect:
- "Irrelevant once the DV is measured" is false: individual differences still matter if they systematically differ across groups.
- "Increase external validity, not internal validity" misrepresents their role; individual differences affect generalizability but uncontrolled systematic differences threaten internal validity.
- "Only matter in correlational studies" is false: they matter in experiments too unless controlled (e.g., by random assignment or blocking).

Final confirmation: The correct answer is "Because random assignment distributes individual differences evenly across conditions, making them unlikely to explain group differences."

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