This week, the overall grades of three Service stocks are lower, according to the Portfolio Grader database. Each of these rates a “D” (“sell”) or “F” overall (“strong sell”).

Sotheby’s (NYSE:BID) is on the decline this week, earning a D (“sell”) after receiving a C (“hold”) last week. Sothebys is an auctioneer of authenticated items, including fine art, antiques, jewelry, and collectibles. As of Feb. 6, 2013, 11.2% of outstanding Sotheby’s shares were held short. For more information, get Portfolio Grader’s complete analysis of BID stock.

The rating of New Oriental Education & Technology Group (NYSE:EDU) slips from C to a D. New Oriental Education & Technology Group offers educational services and training. The stock gets F’s in Earnings Growth and Earnings Revisions. The stock price has dropped 9.8% over the past month, worse than the 3.4% increase the S&P 500 has seen over the same period of time. For a full analysis of EDU stock, visit Portfolio Grader.

Corinthian Colleges (NASDAQ:COCO) earns an F (“strong sell”) this week, moving down from last week’s grade of C (“sell”). Corinthian Colleges is a post-secondary education company in North America. The stock gets F’s in Earnings Revisions and Earnings Surprise. Investors seem to agree with the downgrade and have pushed down the share price 15% over the past month. As of Feb. 6, 2013, 30.5% of outstanding Corinthian Colleges shares were held short. The stock has a trailing PE Ratio of 246.00. To get an in-depth look at COCO, get Portfolio Grader’s complete analysis of COCO stock.

Louis Navellier’s proprietary Portfolio Grader stock ranking system assesses roughly 5,000 companies every week based on a number of fundamental and quantitative measures. Stocks are given a letter grade based on their results — with A being “strong buy,” and F being “strong sell.” Explore the tool here.