Final answer:
A one-group, posttest only research design can describe outcomes after an intervention but cannot firmly establish cause-and-effect due to the lack of a control group for comparison.
Step-by-step explanation:
The one-group, posttest only research design can only describe the effects of an intervention without comparing it to a control group. In this design, all participants are subjected to the treatment, and outcomes are measured afterward. However, because there is no control group, any observed changes could be due to factors other than the treatment.
To establish a cause-and-effect relationship, we need to control for potential confounding variables. This is typically done in an experimental design where subjects are randomly assigned to treatment or control groups, where the only difference is the experimental manipulation.
Only through such a controlled experiment can we infer that differences are indeed due to the manipulation and not to other external variables.