What can cause a dramatic ECPM drop?

[rick sample]

I'm not going to say the exact amounts since it might be against googles TOS. But the last 3 days, I've seen about $3 less on ECPM. What causes that to go up and down?

[Liminal]

1) One or more of the best-paying advertisers cease to advertise (perhaps they reached their budget limit for the month).
2) G adjusted their payout structure

[alang]

conversion rate low. Most of visitors that clicks ads on your site, didn't buy anything from advertiser site thus, they lower the ECPM on your website.