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A true experiment requires the researcher to actively intervene and manipulate the independent variable; merely comparing pre-existing groups does not constitute manipulation. For instance, comparing the health of people who already keep a journal with those who do not is not an experiment, because these groups likely differ in other confounding ways (such as conscientiousness or stress levels). Active manipulation is crucial for eliminating these potential alternative explanations for the results.

Active Manipulation in Experiments

There are many situations where a researcher cannot actively manipulate an independent variable for practical or ethical reasons, making a true experiment impossible. For example, a researcher cannot ethically manipulate whether individuals suffer a significant early illness to study its effect on hypochondriasis. When active manipulation is not possible, researchers must investigate the relationship using nonexperimental approaches.

Inability to Manipulate Variables

What is a critical requirement regarding the independent variable for a study to be classified as a true experiment?

A researcher wants to study whether journaling improves mental health. She recruits 200 participants and finds that 95 of them already keep a daily journal while the remaining 105 do not. She measures each group's mental health scores and compares the results. This study qualifies as a true experiment because it compares two distinct groups on a dependent variable.

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