I have to do some more reading before I can give you a definitive answer. The papers that I have as background have methodology sections, but so far, I have only been skimming the results and conclusions...so its gonna take more time....sorry I could be of more help at this time.

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Ride hard, Ride long....git put away Wet each and every time....no holdin' Back.....enjoy the journey ...... cuz that's ALL there is! Walk on and walk tall folks!