Question

the difference between an observational study and an experiment is that
in an observational study, only one group is studied, and in an experiment, two groups are studied.
in an observational study, the researchers do not control treatment, and in an experiment, they do.
in an experiment, cause-and-effect is analyzed, and in an observational study, it is not.
in an experiment, one group is studied over a short period of time, and in an observational study, the group is studied
over a longer period of time.

Explanation:

To determine the correct answer, we analyze each option:

• Option 1: Observational studies and experiments can involve one or more groups, so this is incorrect.
• Option 2: In an observational study, researchers observe without controlling treatments (e.g., observing disease rates in a population without intervening). In an experiment, researchers control the treatment (e.g., assigning a drug or placebo). This matches the key difference.
• Option 3: Both observational studies and experiments can analyze cause - effect (though observational studies have more confounding factors), so this is incorrect.
• Option 4: The duration of study isn't a defining difference between observational studies and experiments.

Answer:

B. in an observational study, the researchers do not control treatment, and in an experiment, they do.