The September unemployment rate was the lowest since December 2008 after surprisingly large back-to-back declines, sending economists back to the drawing board after big forecast misses. Some pointed to the large increase in involuntary part-time employment – erroneously so, according to an analysis from Ray Stone, economist and managing director at Stone & McCarthy.

The jobless rate fell to 7.8 percent last month from 8.1 percent in August.

After a quick, superficial look at the September household data, several commentators embraced the thesis that it was due to a 582,000 increase in Part-Time Employment for Economic Reasons. These are people who prefer full-time employment, but sadly had to settle for a part-time job. These 582,000 part-timers accounted for much of the overall 873,000 increase in September civilian employment.

This Part-Time for Economic Reasons statistic “was a Greenspan favorite, and certainly over longer periods of time such is a measure of labor distress,” Stone said. “But, the month-to-month wiggles in this series usually turn out to be noise. In September this metric rose to 8.613 million.”

Especially interesting is that each of the last three Septembers saw similar surges in the Part Time for Economic Reasons statistic and that the size of the September 2012 increase was similar in size to the previous  three Septembers when nothing particularly noteworthy occurred to the unemployment rate.

In that context, “is it fair to attribute the 0.3 percent decline in September 2012′s unemployment rate to a surge in the Part-Time for Economic Reasons statistic?” Stone asks.