Find the Publication Type in the lower right and select, Reports- Research
Form the advanced search page, find the Metholody section in the lower right hand side. Choose Empirical Study
Since these are published in Scholarly Journals, limit to Academic, Scholarly, or Peer reviewed articles.
Then try adding this boolean phrase to your search empirical or study or studies.
This will help limit your results but does not guarantee that all articles will be empirical studies.