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) - Get Report

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.

Georgia-based

United Community Banks

(UCBI) - Get Report

has been upgraded to buy from hold. The company's revenues grew 28.5% in the fourth quarter of fiscal 2006 over the year-earlier period, outpacing the industry average of 3.4%. Earnings per share and net income also rose over the same period. United Community Banks had been rated hold since March 2005.

Reinsurance provider

Renaissancere Holdings

(RNR) - Get Report

has been downgraded to hold from buy. The company's revenues have fallen over the past year, and TheStreet.com Ratings expects Renaissancere's EPS growth to slow going forward. The company had been rated buy since September 2006.

Goodrich Petroleum

(GDP) - Get Report

has been downgraded to hold from buy. The company is involved in oil and gas exploration and production in eastern Texas and northwestern Louisiana. Goodrich's net income swung to a loss in the fourth quarter of 2006 compared with the same quarter a year ago. Earnings per share have also declined. The company had been rated buy since November 2006.

Semitool

( SMTL) has been upgraded to buy from hold. The company produces equipment for the manufacture of semiconductors. Semitool has seen robust revenue growth over the past year, and remains in a solid financial position with relatively little debt. The stock price has gone up more than 6% in the past year, and TheStreet.com Ratings sees more upside potential in the future. Semitool had been rated hold since March 2005.

Additional ratings changes are listed in the table below.