For many observational independent variables,only between-subjects designs are applicable.

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Observational independent variables refer to variables that cannot be manipulated by the researcher, such as age, gender, and race. In some cases, it may be possible to use within-subjects designs to study these variables (e.g., by measuring changes over time within the same individuals). However, for many observational independent variables, only between-subjects designs are applicable.

For Many Observational Independent Variables,only Between-Subjects Designs Are Applicable