NEW YORK (CNN/Money) - A University of Michigan researcher who runs a well known survey which measures customer opinion about popular products and services, said he has been buying and short selling the stock of some companies in the index, sometimes before the study is released, according to a published report Tuesday.

Claes Fornell oversees the American Customer Satisfaction Index. He said Friday he took out short positions on Home Depot and McDonald's, two companies that saw their satisfaction rates decline in the latest survey to be released Tuesday, the Wall Street Journal reported.

An investor takes a short position in a stock by borrowing shares and selling them with promise to return them to the lender at a later date. The investor hopes the shares will decline, and he can pocket the difference between the sale and repurchase price.

Fornell also said he bought shares in Costco Wholesale Corp. and Yum Brands Inc., two companies with improving satisfaction rates in the latest survey, according to the paper.

"As soon as I have some information on [customer-satisfaction scores] I make these trades," Fornell said, adding that he thought it was important to test his theories on the survey with real money. Fornell said his ACSI-based portfolio, which includes about 30 stocks, fell 5 percent to 6 percent since he started trading it in April of 2000, according to the Journal.

In the same time the Dow Jones Industrial Average fell about 30 percent.

Securities lawyers say Fornell's trading is a gray area of the law and its legality stands on whether the study itself has a material impact on share prices.

In a March 2001 article in the Harvard Business Review, Fornell said the stock of companies with high customer-satisfaction scores tended to outperform those with lower scores. He added Friday he doesn't believe the University of Michigan's research has any impact on share prices, according to the paper.

Nancy Connell, a spokeswoman for the University of Michigan said the institution is looking into Fornell's stock trading. "This is a complex set of facts," she said. "Until we know more about it all we can say is we are reviewing the relevant policies that might apply."