The overall ratings of three Service stocks are down on Portfolio Grader this week. Each of these rates a “D” (“sell”) or “F” overall (“strong sell”).

Apollo Group (NASDAQ:APOL) ratings are on the decline this week as the company earns a F (“strong sell”). Last week, it received a D (“sell”). Apollo Group is a private-education provider at the undergraduate, graduate, and doctoral levels. For Portfolio Grader’s specific subcategory of Sales Growth, APOL also gets an F. The stock price has fallen 31% over the past month, worse than the 6% decrease the Nasdaq has seen over the same period of time. As of Oct. 24, 2012, 13.9% of outstanding Apollo Group shares were held short. For more information, get Portfolio Grader’s complete analysis of APOL stock.

This week, Collectors Universe (NASDAQ:CLCT) falls to a D (“sell”), worse than last week’s grade of C (“hold”). Collectors Universe provides third-party authentication, grading, and related services for rare and collectible items. Share prices fell 13% over the past month. For a full analysis of CLCT stock, visit Portfolio Grader.

The rating of ATA (NASDAQ:ATAI) declines this week from a C to a D. ATA offers services for the creation and delivery of computer-based tests based on its proprietary testing technologies and test delivery platform. The stock also gets an F in Earnings Momentum. To get an in-depth look at ATAI, get Portfolio Grader’s complete analysis of ATAI 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.