Higher values mean higher payments. Medicare has assigned a numerical value to each of more than 7,000 services and procedures. Officials multiply this number by a “conversion factor” — now about $36 — to determine how much doctors will be paid for a service.

While changes in the value of particular services are not supposed to alter the amount spent by Medicare, they can determine winners and losers, by increasing payments for some services at the expense of others.

Dr. Barbara S. Levy, who has been chairwoman of the medical association’s update committee for the last six years, defended its work and said she did not see any conflicts of interest.

“We are not talking about dollars or money,” said Dr. Levy, a gynecologist. “We are talking about the time and resources that are necessary to perform a procedure, including: How many sutures does it take? And what sort of equipment? And how many minutes of my nurse’s time? And do I need a nurse versus a medical assistant for the safety of my patient?”

“I can’t imagine how anyone other than a group of physicians could provide that kind of expertise,” she said.

Another committee member, Dr. Gregory J. Przybylski, a neurosurgeon in New Jersey, said, “It is very difficult to get information about services being provided unless you ask the people who are actually providing those services.”

Representative Jim McDermott of Washington, the senior Democrat on the Ways and Means subcommittee on health, said: “Medicare certainly needs clinical expertise to appraise the value of doctors’ services, but we give medical specialty societies an undue influence on their own payments. Medicare is a cash cow for specialists and not for family practitioners.”