Each weekday, TheStreet.com Ratings updates its ratings on the stocks it covers. The proprietary ratings model projects a stock's total return potential over a 12-month period, including both price appreciation and dividends. Buy, hold or sell ratings designate how the Ratings group expects these stocks to perform against a general benchmark of the equities market and interest rates.While the ratings 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 company earnings forecasts. Objective elements include volatility of past operating revenue, financial strength and company cash flows. Western Refining ( WNR), an independent crude oil refiner, has been upgraded to hold. While the company has enjoyed notable return on equity, robust revenue growth and attractive valuation levels, its debt management and profit margins have been poor. The company earned $155.0 million, or $2.29 a share, for the second-quarter, up from $86.5 million, or $1.29 a share, a year ago. Powered by its strong earnings growth and other factors, Western Refining's stock has surged by 50.6% over the past year, but TheStreet.com Ratings does not recommend additional investment in this stock despite these gains. Western Refining had been rated sell since February. Thornburg Mortgage ( TMA), a single-family residential mortgage lending company, has been downgraded to sell. The company's weaknesses can be seen in several areas, such as its feeble EPS growth, deteriorating net income, weak debt management, disappointing return on equity and generally unsatisfactory historical stock performance. Thornburg recently reported a third-quarter loss of $1.09 billion, or $8.83 a share, compared with earnings of $72.9 million, or 64 cents a share, a year earlier. EPS has declined over the last two years and this is expected to continue in the coming year. The stock has tumbled 62.6% over the last year and return on equity greatly decreased from a year ago. Thornburg Mortgage had been rated hold since October 2006.