A non-experimental study is one that lacks manipulation of independent variables, control for irrelevant variables by random assignment, or both.
Random assignment to order of conditions in between-subjects experiments or conditions in within-subjects experiments is a fundamental element of experimental studies. Its purpose is to control the external variables from becoming noise variables.
To control for participant variables, we should aim to randomly divide the sample into control and experimental groups. Random assignment distributes participant characteristics evenly across groups, thus making the groups comparable.
In theory, because people are randomly assigned, the effects of high-risk irrelevant variables are evenly distributed across different groups, thus controlling for confounding effects.
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