Consider a thought experiment: Suppose countries A and B each have 10 people employed and that output per worker in the two countries is equal. Workers within each country are not equally productive but the distribution of productivity is the same in both countries. (If it is easier to grasp--assume 10 sets of identical twins have been separated at birth with one child from each set of twins going to each country.)

Now suppose that country B decides to enact some sort of tax or labor market regulation. As a result of this policy the least productive worker in country B is no longer employed. (For concreteness, the policy might be a minimum wage law that makes it unprofitable for a firm to hire the worker at the minimum wage.)

What effect does this policy have on the productivity of workers in the two countries? Country B's productivity now appears to be higher than country A's because the least productive worker in country B is no longer employed and no longer counts in the productivity calculations. (This is akin to having one's course average increase when the lowest grade is dropped.)

So, professor, where are you going? To Paul Krugman's column in today's NYT:

"First things first: given all the bad-mouthing the French receive, you may be surprised that I describe their society as "productive." Yet according to the Organization for Economic Cooperation and Development, productivity in France - G.D.P. per hour worked - is actually a bit higher than in the United States."

The key part here is per hour worked. The difference in employment between the two countries is quite large--France's 2003 employment to population ratio was 51.9% while it was 62.3% in the U.S. I bet that French output per working age adult--that is adjusted in some manner for people who are out of work--is lower than in the U.S. Stated differently, I bet the most productive 51.9% of the U.S. population is more productive than the 51.9% of the French who are employed.* In other words, I think Krugman and the OECD comparing apples to oranges when claiming that French productivity exceeds U.S. productivity.

*I'm implicitly assuming that differences in the employment-population ratios are due entirely to taxes and labor market regulations. There may well be some other factors (e.g., demographics) that account for part of the difference, but I don't think these factors would alter my conclusion.