Contrary to public perceptions, the National Security Agency's controversial program to collect information on phone calls to, from and within the U.S., gathers such metadata on only a small percentage of U.S. telephone traffic, a member of President Barack Obama's surveillance review group said Thursday.

"In fact, the NSA collects a very small percentage of our phone logs," University of Chicago Professor Geoffrey Stone said during a symposium sponsored by the advocacy group Public Citizen. "The numbers it collects are effectively randomly collected…They do it by service provider. If you happen to use a service provider who participates in their program, you're number might get picked up. If you use another company, your number won't get picked up."

Stone did not specify which providers are involved in the program, which is aimed at preventing terrorist attacks. He also said he could not disclose the precise percentage of calls about which the NSA collects data.

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