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”).

National American University Holdings (NASDAQ:NAUH) ratings are on the decline this week as the company earns a F (“strong sell”). Last week, it received a D (“sell”). National American University operates National American University locations throughout the central United States. In Portfolio Grader’s specific subcategories of Earnings Growth and Margin Growth, NAUH also gets F’s. The trailing PE Ratio for the stock is 27.60. For a full analysis of NAUH stock, visit Portfolio Grader.

This week, American Public Education (NASDAQ:APEI) falls to a D (“sell”), worse than last week’s grade of C (“hold”). American Public Education provides online postsecondary education focusing on the needs of the military and public service communities. The stock price has fallen 7.1% over the past month, worse than the 0.6% decrease the Nasdaq has seen over the same period of time. As of Nov. 28, 2012, 18.2% of outstanding American Public Education shares were held short. To get an in-depth look at APEI, get Portfolio Grader’s complete analysis of APEI stock.

Xueda Education Group (NYSE:XUE) experiences a ratings drop this week, going from last week’s C to a D. Xueda Education Group provides tutoring services for primary and secondary school students in the People’’s Republic of China with a focus on offering personalized tutoring services. The stock gets F’s in Earnings Revisions and Margin Growth. The stock’s trailing PE Ratio is 28.00. For more information, get Portfolio Grader’s complete analysis of XUE 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.