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 forecasted company earnings. Objective elements include volatility of past operating revenue, financial strength and company cash flows.

Some recent rating changes are highlighted below.

TheStreet.com Ratings has initiated coverage of aircraft component manufacturer

Transdigm

(TDG) - Get Report

with a sell rating. The company has a debt-to-equity ratio of 2.39, which is higher than the industry average. Transdigm's stock trades at 53 times earnings, a significant premium over shares of its peers, which have an average P/E of 18.

Helsinki, Finland-based paper products manufacturer

UPM-Kymmene

( UPM) has been upgraded to a buy from a hold. The company's revenues rose 29.8% in the fourth quarter of fiscal 2006 and appear to have boosted earnings per share. Net operating cash flow increased significantly over that same time period, more than 150%. UPM-Kymmene had been rated a hold since December 2006.

Casual dining restaurant chain

Red Robin

(RRGB) - Get Report

has been upgraded to a buy from a hold. The company has demonstrated a pattern of positive earnings per share growth over the past two years, including a 60.6% increase in the fourth quarter of fiscal 2006 compared with the same quarter a year earlier. TheStreet.com Ratings expects this trend to continue. Red Robin's average cash flow growth rate over the past year exceeded the hotel, restaurant and leisure industry average. RRGB had been rated a hold since January 2007.

Biopharmaceutical firm

Abraxis

( ABBI) has been upgraded to a hold from a sell. Revenue and net income growth exceeded the industry average over the past fiscal year. Abraxis has a debt-to-equity ratio of just 0.23, implying that there has been very successful management of debt levels. ABBI had been rated a sell since August 2006.

Satellite television provider

EchoStar

(DISH) - Get Report

has been upgraded to a buy from a hold. The company's earnings per share increased 16.7% in the fourth quarter of fiscal 2006 compared with the same quarter a year earlier. The stock price has shot up 56.5% in the past 12 months. TheStreet.com Ratings considers EchoStar's 45% gross profit margin to be strong. The company had been rated a hold since April 2005.

Additional ratings changes are listed in the table below.