Determine which of the described results stemmed from an experiment rather than an observational study.

• The percentage of adults who say a Spotify study music playlist helps them increase concentration while studying.
• Netflix's total library size of movies for different countries.
• The short-term effectiveness of the Covid-19 vaccines using a vaccinated and a non-vaccinated group.
• The number of adoptable dogs categorized by their size from May 2005 to December 2019.