This video may be, at first glance, about observational and designed experiments. But really it's a how-to for your future career as a data-mastering statistics master who will not -- who dare not -- do experiments that suck (statistically speaking). God speed.
Prospective (Longitudinal or Cohort) vs Cross-Sectional vs Retrospective Studies
With these studies, it's mostly a matter of timing. Discuss.
Double-Blind, Placebo-Controlled Study
This term mostly applies only to medical research trials, such as investigating new drugs and whatnot, but it's illustrative to show you just how awesomely statistical a study can be. No confounding variables here!
If you do not have an account, you should get one, because it is awesome! You can save a playlist for each test or each chapter, and save your "greatest hits" into a "watch right before the final" list (not that we recommend cramming, but when in Rome...)