Today it's harder to rely on share volume as a stock-market indicator than it used to be, Tim Mahoney, chief executive of BIDS Trading, explains to MarketWatch's Greg Morcroft. Mahoney says we are witnessing a transformation in the marketplace as “the role of the market maker has moved from a person to a computer.” Mahoney estimates that 70 percent of the volume in the markets today comes from computerized trading. He also discusses how the fragmentation in the markets is skewing the traditional volume numbers — from NYSE and Nasdaq — that investors traditionally relied on.