Forum Moderators: martinibuster
If you're confident the numbers you're talking about are statistically significant, take a look at the ads showing. If a change in ad targeting has really dropped CTR by a factor as massive as 80%, the ads should be markedly different.
[Just noticed you're saying ads are the same. I'm going to fall back on the numbers probably not being statistically significant; possibly related to the fact that you posted very early into the new reporting day.]
If your page quality or site quality seems low then as GreenGrass said earlier they may send you lower click through rate or lower paying ads.
During this same period I and some others seem to have had a small increase in both!
But natural statistical variations mean you cannot see any meaningful trends daily unless you normally get a good few hundred clicks per day anyway. And even then, its still subject to the usual external effects of advertising budgets, the auction system, hols, events, seasonal stuff, TV, new links appearing or vanishing, search quality / quantity as updation occurs on different engines etc.