here is a study, I'm sure there are more, as for credible, that is for you to decide. http://web.ebscohost.com/ehost/pdfviewe ... d=1&hid=12
from the abstract
The total flavavol content varied from 21.2 to 103.2 mg/g for regular teas and from 4.6 to 39.0 mg/g for decaffeinated teas. The ORAC value varied from 728 to 1686 trolox equivalents/g tea for regular teas and from 507 to 845 trolox equivalents/g for decaffeinated teas.
as for method of decafing, widely used solvent ethyl acetate is the most common form of decaf, and supposedly strips a lot more then co2 extraction. If anyone is really curious I'm sure online literature is out there. I could go look for it if you want me to.
and 2013 USDA measurements for the flavonoid content of select foodshttp://www.ars.usda.gov/Services/docs.htm?docid=6231
I've gone through the tables, basically confirms what we know, I am assuming the method was ethyl acetate since differences were so huge
You can browse them yourself, open it in Microsoft access, go to FD_GROUP, then click BEVERAGES and voila
of potential interest is comparisons that they did with different waters, would be interesting to see how/if any synergy is lost with changes in water, but for some reason I can not find that table again after a quick re-browse...another day