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 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. Civil and defense airplane manufacturer Empresa Brasileira de Aeronautica ( ERJ) has been downgraded to a sell from a buy. Net income decreased 59.9% in the first quarter of 2007 compared with the same period last year. While its share price has jumped 52.9% from its closing price one year ago, the sharp appreciation is one of the factors that should prompt investors to seek better opportunities elsewhere. The company saw a steep EPS decline in the first quarter of 2007 compared with the same quarter in 2006 -- a trend that has been going on for the past two years. Empresa Brasileira de Aeronautica had been rated a buy since October 2006. Nationshealth ( NHRX) provides medical products and prescription-related services to Medicare and managed-care beneficiaries. It has been downgraded to a sell from a hold. Despite that the share price has gone down 45.2% in the past year, the stock is still more expensive (when compared with current earnings) than most other companies in its industry.