You might discover that buttons drive more clickthroughs to your site than text-based links or vice versa.

http://epicentro.instructure.com/eportfolios/2256/Seroflo_Rotacap_100_Mcg_Power__Cipla__Asthma
http://eou.instructure.com/eportfolios/1702/Dulester_30_Mg_Side_Effects__ILKO__Antidepressants

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Or try sending each variation of the test to only a portion of your list over a defined period of time and then send the best-performing version to the rest of your list afterwards.