I doubt anyone ever noticed since no one has ever commented, but I apply a hierarchy when it comes to quoting sources to back up my statements. Nothing undermines and argument like a dubious source, and who I quote might say something about me (e.g. Naomi Klein, yes; Naomi Wolf, no).
I always try to seek first hand sources. If someone is quoting another, go back to that quoted source – because sometimes, even they are quoted sources. Quotemining, biases and agendas, misunderstanding, “interpretation”, or a game of telephone – they all detract from what you’re trying to say.
Below the fold is a rough and very incomplete list of sources by what I consider their level of credibility. Those named are not the only source I would use, and this applies to any subject or science (even though many listed are medical). The groups listed near the top are interchangeable, several being equally credible. Those at the bottom (less credible or fact checked) being interchangeable for credibility.