Friday's much-anticipated jobs report could be the final piece of the Federal Reserve puzzle for raising interest rates.

Should the number come in considerably above expectations of about 220,000, many market participants believe the U.S. central bank will vote to raise interest rates for the first time in more than nine years.

To whatever extent the number shows continued solid job gains, however, there likely will be relatively little consideration given to a less appealing state of the employment picture.

A fresh look released Thursday provides some perspective: For all the talk about the nearly 250,000 jobs a month the economy is creating, workers' real wages, including the cost of living, are going backward. Average pay in real terms slumped 4 percent from 2009-14, according to the National Employment Labor Project.

Read More Fed official: No evidence QE boosted economy



What's more, the jobs that have been most plentiful during the post-Great Recession boom have seen some of the biggest declines in pay. Restaurant workers, whose ranks have swelled by 376,000 over the past year (according to the Bureau of Labor Statistics), saw real pay declines of 8.9 percent for cooks, 7.7 percent for food preparers and 4.8 percent for waiters and waitresses.