“To the extent you would have been expecting anything, it would have been down,” Eidlin said. “We’ll have to see whether this is part of a trend. That is still up in the air.”

The numbers, drawn from a monthly survey of about 60,000 households, show that overall about 7.3 million public sector employees and 7.2 million in the private sector belonged to unions in 2013.

That translates to a rate of about 35 percent for public sector workers, but just 6.7 percent for those in the private sector.

Overall, protective service and education are the occupations with the highest union membership rates, while sales, farming, fishing and forestry are the lowest. Among private sector industries, transportation and utilities are the most unionized.

The report shows Wisconsin had an increase of about 24,000 union members in 2013, while the overall number of wage and salaried workers dipped by about 36,000.

James Sherk, a senior policy analyst in labor economics for the conservative Heritage Foundation, said the biggest statistically significant change was in the financial sector, with some marginal increase in wholesale and retail trade unionization.

Even one union opponent was unable to explain the change.

“Right now it’s just interesting,” said David Denholm, president of the Public Service Research Foundation, an independent research organization focused on union influence on public policy. “There’s no pat explanation for it, but it’s worth looking at.”

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