From the summary:
The Great Recession caused labor market devastation on a scale not seen for many decades. Millions of jobs were lost in the United States during 2008 and 2009, leaving the labor market with a hard road to recovery. Indeed, that recovery has required many years of job growth, and it was only in April 2014 that total employment reached its pre-recession level.
However, this milestone did not mark a return to pre-recession labor market conditions. Because the U.S. population is growing, simply reaching the previous number of jobs is not sufficient to return to pre-recession employment rates. At the same time, more baby boomers have entered retirement, somewhat offsetting the effects of population growth and reducing the number of jobs needed for a full economic recovery.
In order to accurately track the progress of the labor market recovery, The Hamilton Project developed a measure of labor market health—the “jobs gap”—that reflects changes in both the level and the demographic composition of the U.S. population (more details regarding the jobs gap methodology are provided in appendix A). Beginning in May of 2010, The Hamilton Project has calculated the number of jobs needed to return to the national employment rate prior to the Great Recession, accounting for population growth and aging.