I want to assess the impact of treatment (advertisement) on consumer behavior. What could be the most suitable experimental design here? Shall a single group pretest - posttest suffice?
I personally would suggest the between-subject design (multi groups) than the within-subject (single group). But of course it would depend on the research aims. If your want to see participants' attitudinal and behavioral changes caused by the treatment(s), I think within-subject is the proper option. But if you want to see different responses from different treatments, than the between-subject will fit the aim.
It’s premature to ask design questions before formulating your research questions and aims. Your research question(s) and aims should drive your efforts. This might shed some light on why: https://rdcu.be/c0iE6