Many good refs define both but [1] is free. As far as how you might think of them [2,3] give context. The exact extent of merit can be debated but these do have merit.
[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5903119/ [2] https://ebm.bmj.com/content/21/4/125 [3] https://jamanetwork.com/journals/jama/fullarticle/1895230