Pfizer (PFE) Chart Analysis

Pfizer staged a surprising rebound when market tumbled more than 2% on 5/4.  What triggered this rebound then? 

The above chart shows that $16.82 is the 50% Fibonacci retracement from October 2007 high to March 2009 low (click to enlarge). 

If we look at the time frame from March 2009 low to Jan 2010 high, then approximately the same level $16.75 is the 38% retracement from Jan 2010 high, according to chart above (click to enlarge)

We should use $16.75 as stop loss.