What are "Levels of Evidence?"
Have you ever seen "Level of Evidence" in an article abstract? Has anyone ever told you that they only want "level 2 or above"?
Here's a few resource: https://www.essentialevidenceplus.com/product/ebm_loe.cfm?show=oxford#accept
The CEBM has great information also. First read this Introductory Document (3 pgs): https://www.cebm.ox.ac.uk/resources/levels-of-evidence/ocebm-levels-of-evidence then review the file below.
There is also excellent information on this Wikipedia page