Marketers must recognize several important things about the stock market and the way Wall Street reacts to changes in outlook for the companies involved in online advertising and marketing. First, we don’t make media placement decisions based on the engines’ stock prices. Wall Street’s investors collectively set stock prices based on their estimates of a company’s future profit and growth. Sometimes, Wall Street is right and sometimes it’s wrong, in both directions. Wild swings in either direction stem from surprises. Those can be positive or negative changes in earnings information or projections or, as in Yahoo’s case, a company’s failure to adhere to a promised release date of a PPC (define) algorithm change that promises higher revenues per million searches.