Final answer:
The simplest experimental design when subjects are randomly assigned is the post-only design with control. It involves an experimental group and a control group, with the latter often receiving a placebo to ensure observed differences are due to the treatment.
Step-by-step explanation:
When subjects are randomly assigned, the simplest experimental design is the post-only design with control (a). This design entails having two groups: an experimental group and a control group. The only difference between the control group and the experimental group is the independent variable being tested. For instance, in testing a new drug, one group receives the drug (experimental), and the other receives no drug or a placebo (control). By comparing outcomes between the two groups after the experiment, researchers can assess the effect of the drug on the subjects.
In contrast, a time series design with control (b), a single group design with control (c), and a pre-post design with control (d) are typically more complex because they involve additional measurements, could require manipulating multiple variables, or entail both pre- and post-treatment assessments.
Experimental controls, like having a control group, are crucial as they allow researchers to compare outcomes between groups that differ in only one independent variable (d). This comparison is essential to assess the influence of the explanatory variable. The control group often receives a placebo to ensure that any differences observed are due to the experimental manipulation, not other factors.
Thus, a post-only design with control is the most straightforward experimental design when subjects are randomly assigned.