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I rarely get PSAs on my sites, except when AdSense is serving PSAs to all publishers due to a problem on their end.
Same here. I don't bother with alternative ads for that reason.
I'd guess that the number of PSAs served depends on:
1) Your topic and subtopics. (A diverse range of subtopics may help to prevent PSAs.)
2) How many sites are competing for ads on your pages' keywords and keyphrases. (If 5,000 sites are competing for a finite number of clicks, advertisers may quickly reach their budget limits.)
3) How many advertisers there are for your pages' keywords and keyphrases.
4) Whether advertisers in your subject area have had good or bad experiences with content ads. (If there are a lot of shady sites in your category, Adwords advertisers may have opted out of content ads.)