Economists within the Federal Reserve are struggling to size up the strength of the U.S. labor market but can’t even agree what yardstick to use. Andreas Hornstein, a senior adviser at the Richmond Fed, says the unemployment rate, currently at 6.3 percent, effectively captures the level of slack in the labor market. To Mary Daly, a top economist at the Fed’s regional branch in San Francisco, a full assessment should go beyond the jobless rate to take into account stagnant wages, the large numbers of long-term unemployed and a drop in labor force participation.