Custom variables are name-value pair tags that you can insert in your tracking code in order to refine Google Analytics tracking.
https://developers.google.com/analytics/devguides/collection/gajs/gaTrackingCustomVariables?hl=en
Analytics and Tracking
Get access to data on 32,414 websites that are Google Analytics Custom Variables Customers. We know of 9,395 live websites using Google Analytics Custom Variables and an additional 23,019 sites that used Google Analytics Custom Variables historically and 4,069 websites in the United States.
Get a list of 32,414 websites using Google Analytics Custom Variables which includes location information, hosting data, contact details, 9,395 currently live websites and an additional 68,123 domains that redirect to sites in this list. 23,019 sites that used this technology previouslyand 4,069 websites in the United States currently using Google Analytics Custom Variables.
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