Question at position 15 The key difference between an observational study and an experiment isThe use of random selection of participantsThe number of variables that are being studiedThe ability to replicate the studyThe creation of groups of homogenous subjects to studyThe assignment of treatments to experimental units

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BlackTom AI · Accepted Answer

To tackle this question, let’s compare the core methodological difference between observational studies and experiments, and then evaluate each option.
Option 1: The use of random selection of participants. In observational studies, researchers typically observe subjects without assigning interventions, and random selection can occur but is not the defining feature. In experiments, randomization of units to different treatment groups is a key feature, but random selection of participants is not the fundamental distinction; both designs can involve or omit random sampling. Therefore, this option captures a methodological convenience rather than the defining difference.
Option 2: The number of variables that are being studied. Both observational studies and experiments can examine multiple variables; the quantity of variables is not what structurally separates them. You might have many variables in either design, so this is not the distinguishing feature.
Option 3: The ability to replicate the study. Replicability is a general scientific principle that applies to both observational studies and experiments. It is not exclusive to experiments, and some observational studies can be difficult to replicate due to ethical or practical constraints. Thus, this is not the defining difference between the two designs.
Option 4: The creation of groups of homogenous subjects to study. Some experimental designs do strive for homogenous groups through randomization or matching, but the use of homogeneous groups is not the defining hallmark of experimental versus observational studies. Observational studies can also involve groups that are compared, and homogeneity is not the core distinguishing feature.
Option 5: The assignment of treatments to experimental units. This option highlights the central difference: in experiments, researchers actively assign an intervention (treatment) to experimental units (such as participants, plots, or samples) and manipulate the independent variable to observe effects. In observational studies, researchers do not assign treatments; they observe existing conditions and look for associations. This assignment-for-causation structure is the defining characteristic that separates experiments from observational studies.
So, considering which option best captures the fundamental difference, Option 5 correctly identifies the key distinction: the assignment of treatments to experimental units. The other options describe practices (randomization, replication, group formation) that may appear in various designs but do not define the critical methodological boundary between observational studies and experiments.

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