TheStreet.com Ratings: Stock Upgrades, Downgrades

Each weekday, TheStreet.com Ratings updates its ratings on the stocks we cover. Our model projects a stock's total return potential over a 12-month period, including price appreciation and dividends. Our buy, hold or sell ratings designate how we expect these stocks to perform against a general benchmark of the equities market and interest rates.

While our model is quantitative, it uses both subjective and objective elements. For instance, subjective elements include expected equities market returns, future interest rates, implied industry outlook and forecasted company earnings. Objective elements include volatility of past operating revenue, financial strength and company cash flows.

Some recent rating changes are highlighted below.

Westlake Chemical(WLK Quote) has been downgraded to hold from buy. The company's revenues fell 17.7% in the fourth quarter of fiscal 2006 compared to the same quarter a year earlier. Gross profit margins also declined over the same period. Westlake had been rated buy since February 2007. ...