Short interest measures the total number of shares of a security that has been sold short, expressed as a percent of total tradable shares.



Investors track short-interest levels to gain a sense of where a stock might be headed, along with some insight into whether any positive news might force short traders to cover their positions, pushing stocks higher.



Barnes & Noble and Diamond Foods, for example, have some of the highest ratios in the S&P 1,500 index, with more than 50 percent of their float sold short.



Since late November, R.R. Donnelley & Sons, Borgwarner and Safeway joined the top 20 most shorted companies in the S&P 500 index, while AK Steel, Ecolab and Moody's have fallen off the list.



Here is a look at the most heavily shorted stocks, along with the percent change in stock price and short interest in the past month.









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