In a recent speech to the National Association of Business Economics, Federal Reserve Chairwoman Janet Yellen discussed the difficulties she and her Fed colleagues face in determining when and by how much to raise interest rates. The challenge stems from the fact that their preferred inflation indicators are heading south at a time and under conditions where conventional wisdom says they should be trucking north.

The Fed’s bright line for inflation heartburn is marked by sustained price level growth around 2 percent annually. Today’s number, at about a half percentage point below that bright line, seems troubling. But meanwhile, the nation is experiencing a growing labor force and low levels of unemployment, which should ordinarily signal more inflation.