Ze'ev - CSFB downgrades.... since CSFB instituted their new rating structure, they have had only four neutral-to-underperform downgrades.
Average 1st-day move... -15%
5th-day........ -20%
10th-day....... -31%
As for your experience that downgrades are used to get cheap stock, I must say that I've seen a few instances where that appeared to be the motivation for the downgrade. However, I've developed a database of up/downgrades and the subsequent price changes, and the data indicates that downgrades are, on average, followed by percentage drops greater than the Market.
Market Data 
Markets 
AI
