Native ads are ranked based on their campaign eCPMi.
A campaign's eCPMi is determined by a combination of its bid and click-through rate (CTR).
The CPMi is recalculated hourly for 7 days rolling (168 hours). It's updated if a campaign had at least 10 clicks or 100.000 impressions over this period (rolling week).
An individual eCPMi is calculated for each publisher and sector, provided the campaign has sufficient data.
Please note that eCPMi is calculated at the campaign level only, not for individual ads.