PPA can be as misleading as CPC or CPM. If you ask for an email, it could be iframed with a more compelling offer. If you ask for a credit card purchase, it could be done with big blocks of stolen credit card numbers. This also does not include covariance between marketing channels aka the attribution model problem (ex. if you blast TV ads for freecreditreport.com then the CPA for online advertising magically drops).
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PPA can be as misleading as CPC or CPM. If you ask for an email, it could be iframed with a more compelling offer. If you ask for a credit card purchase, it could be done with big blocks of stolen credit card numbers. This also does not include covariance between marketing channels aka the attribution model problem (ex. if you blast TV ads for freecreditreport.com then the CPA for online advertising magically drops).