Which seems like an obvious question but I thought I would ask it all the same.
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CPM is what advertiser pays per 1000 views. Rate changes depending on what advertiser is willing to pay.
All of my non-featured hubs have adverts though so... what does that mean?
It means it has ads, but it no longer shows up in search engines. No one can see your article unless they go to your personal HP profile and maybe not even then. So CPM would be irrelevant if no one can view your hub.
Are you sure? Featured means it is on a search engine and unfeatured means it is not. A view via a search engine has a higher value CPM.. or so I was told.
CPM is what advertisers pay.per 1000 impressions.