S.G.R. More than 99 percent of Americans have no idea what these three letters stand for. And yet they are extremely valuable: worth about $140 billion. This week, a House committee will finally take up the issue. The S.G.R., or the Sustainable Growth Rate formula, was enacted as part of the Balanced Budget Act of 1997 to restrain the inevitable increase in Medicare's annual spending on physician services. It set a reasonable target for each year's increase, and Medicare was supposed to spend less than that amount. If it failed, then the per-service payments to physicians in the following year would be reduced to hit the target. Nice idea in theory. Never worked out in practice.