(Bloomberg Business) — If you're working multiple jobs just to make ends meet, Federal Reserve officials are looking for you.

Central bankers considering when to execute the first interest-rate increase in almost a decade are weighing the extent of hidden slack in the labor market—the kind of underutilization of labor resources that isn't captured by indicators like unemployment and labor-force participation rates. One possible source: the million-plus Americans cobbling together two or three jobs for hours that add up to a normal workweek. 

The share of the employed population working multiple part- time jobs for full-time hours has been rising for 11 of the last 12 years, reaching 0.78 percent in 2014, a record in two decades of data. These people are not included in statistics on the part-time population, because the Bureau of Labor Statistics defines full-time workers in its current population survey as "those who usually worked 35 hours or more (at all jobs combined)." Counting them as fully employed, especially when they would rather have just one steady job, may mask weakness in the labor market.