Mysterious CPM drop yesterday, back to normal now

[robsynnott]

Well, as it says, really, CPM, which is normally stable to within 20 cents, fell hugely yesterday. Ordinary impression count and clicks. It's back to normal now. I'm not really overly concerned about this, but has anyone any idea why it'd happen?

[Pengi]

eCPM could drop for various reasons you need to look at all the data to see what is going on.

If, for some reason, Ads on your site go off target, your click through rate would fall and this would cause eCPM to drop. In the extreme case, you may be showing PSAs and eCPM would fall to zero.

If CTR has remained constant, then that suggest that your earnings per click must have fallen. This could happen if you exhaust the advertising budgets for the better paying Ads, if some advertisers paused their Ads, or if you are being smartpriced.

Smartpricing is unlikely IMHO to have produced a short term sharp drop as you describe.

My guess is that there has been a change in your advertiser behaviour - maybe a significant advertiser paused a campaign or reduced bids for a short period.

[leadegroot]

The most likely explanation is that your top paying advertiser paused their campaign.
This not only removed their top bid, but let the 2nd leader cpc dropped down to just beat the 3rd leader.
If the top couple of advertisers are in a bidding war that is significantly higher than the rest of your advertisers then this would explain it nicely.
This is also a problem for you - your income is very vulnerable to this advertiser leaving.