Through random assignment of participants to treatment conditions, the investigator manipulates an independent variable and examines its effect on a dependent variable. Can be conducted in the laboratory or in the natural environment.

Permits inferences about cause-and-effect relationships.

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When conducted in the laboratory, findings may not generalize to the real world. In field experiments, control over the treatment is usually weaker than in the laboratory. In natural, or quasi-, experiments, lack of random assignment substantially reduces the precision of research.



The investigator studies the same group of participants repeatedly at different ages.

Permits study of common patterns and individual differences in development and relationships between early and later events and behaviors.

Age-related changes may be distorted because of biased sampling, selective attrition, practice effects, and cohort effects.