Now, here’s a case where seasonal adjustments are badly trailing developments in the real world as the recent increase in online purchases around the holidays – and the resulting delivery of those goods – has, presumably, caused the hiring of couriers to spike along with other holiday-related activities in December.

Here’s what the Labor Department’s “Couriers and messengers” subcategory looks like before and after seasonal adjustments are applied to the data:

Seasonal adjustments are supposed to “smooth” out these regular trends, but, for this data series, they are failing badly. Unfortunately, this is not good news for the jobs data that will be reported a month from now because, as shown above for both  2010 and 2011, those outsized seasonally adjusted job gains for couriers in December suddenly turn into big job losses – 40,800 and 48,700, respectively – which should put a damper on the January data.

The Labor Department reported that nonfarm payrolls in the U.S. increased by 200,000 in December, the biggest gain in three months and the fourth best gain for all of 2011, as the unemployment rate fell from 8.6 percent to 8.5 percent.

After holding steady at about 9 percent during most of 2011, the jobless rate has now declined by 0.6 percentage points in just the last three months as the number of unemployed persons has fallen from 13.9 million to 13.1 million. During that same time, the size of the labor force has decreased from 154.0 million to 153.9 million, reducing the participation rate from 64.1 percent to 64.0 percent.

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