Forrester Research announced today that it is creating an online retail index with the National Retail Federation. The index, set to debut in October, will examine online retail category-by-category, providing sales figures for such areas as books, games, and CDs. Used to forecast online retail trends, the index will include data from brick-and-mortar and catalog retailers with more than $100 million in offline sales and Net retailers with more than $500,000 in online sales. A representative for Forrester said the Cambridge, Massachusetts-based technology research firm expects more than 200 companies to participate in the survey by next year.