Promo-code attribution mostly measures Google, not the show
Quote
And it has nothing to do with really the show. It's just got to do with your awareness of the show. And then what how much Google juice has
Episode 139: Rustic Code Cave (episode page, opens in a new tab) 00:20:19 (play audio from this timestamp, opens in a new tab)
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Summary
The same advertiser runs on five shows with five codes. When you finally sign up, you use whichever code ranks when you search, so the code reflects SEO, not which show actually made you aware.