Where can I find data on these "keyed returns" esteemed copywriters speak of in all the most highly recommended copywriting books?

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Hey guys, In my readings (as of right now, How to Write a Good Advertisement, The Robert Collier Letter Book, and Scientific Advertising), the authors of these books keep speaking on "keyed returns," or, as I understand it, in a modern sense relating to digital marketing and advertising, indicators of ROI/ROAS for specific ads and campaigns. How can I, as a beginner, begin researching modern products and services and deconstructing ads that have stood the test of time with measurable results? I'm not just talking about successful swipe files since I'm already in the process of developing a comprehensive one. I'm referring to getting the hard numbers behind some of these ads, if that sort of information is even available to the public. I guess I'm asking if any of you are aware of any specific tools, platforms or methods I'm not aware of myself yet. Thank you all in advance!   submitted by   /u/HollowedYN [link]   [comments]