By the Numbers

S&P's Most Shorted Stocks

Short interest in United States Steel is up 19 percent since late September, with one-fifth of the company's float sold short. In the past seven months, US Steel stock is down 65 percent.

Among the 20 most shorted stocks in the S&P 500, JC Penney has seen the largest decrease in short interest, down 13 percent in a month.  JC Penney stock is up 19 percent during that period.

First Solar and Sears continue to have the highest short-interest ratio, standing at more than 30 percent, according to the latest figures compiled by CNBC. 

Short interest measures the total number of shares of a security that have 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.

Ecolab, Best Buy and Washington Post have joined the top 20 most shorted companies since late September.  By comparison, Novellus Systems and Federated Investors have fallen off the list along with Cephalon, which was acquired by Teva Pharmaceutical for $6.8 billion and no longer trades.

Below is a list of the most heavily shorted stocks along with the percent change in stock price and short interest in the past two months.