Which design corresponds to a classic experimental design?

The classic experimental design is represented by the notation that includes random assignment (indicated by R) and observes a difference before and after an intervention or treatment (indicated by O1 and O2). In the design denoted by R O1 X O2, the 'R' signifies that participants are randomly assigned to conditions, which is crucial for controlling extraneous variables and ensuring that the groups are comparable.

The 'O1' represents the pre-test measurement, assessed before the treatment 'X' is applied. After the treatment, 'O2' denotes the post-test measurement. This structure allows researchers to determine if any changes observed between O1 and O2 can be attributed to the treatment, thus establishing a cause-and-effect relationship.

In contrast, other designs do not incorporate random assignment, pre-tests, and post-tests in the same manner, which is critical for a classic experimental approach. For example, designs that lack randomization or present observations without clear treatments do not meet the criteria to be classified as classic experiments.