I've been following the recent dispute over the proposed Orange County airport with some interest. Particularly interesting to me has been the widespread use of the projected annual economic benefits -- "$40 million to $53 million" -- that a new regional airport would contribute to the local economy. The source of this figure is a recent report by the engineering consulting firm Talbert & Bright.

As an economist who has considerable familiarity with economic impact assessment methods, I'm continually amazed at the way in which numbers such as these insinuate themselves into public policy debates.

The airport controversy is no exception. For example, a recent article in this newspaper quoted Rep. Bill Faison, a strong supporter of building a new airport, as saying, "If you can invest $50 million and get that much back on an annual basis, you'd be crazy not to do it."

Rep. Faison's logic would be unassailable except for one important fact: A careful reading of the Talbert & Bright study reveals that the $40 million to $53 million figure was arrived at via questionable methods and may well be wildly overstated.

There are three serious problems with the study that lead me to this conclusion. First, the projected economic impacts are actually the average impacts of general aviation airports in eight other counties deemed most similar to Orange County on a variety of economic and demographic criteria -- population, unemployment rates, education levels among others.

This is an awfully flimsy basis upon which to assert likely economic impacts of a new Orange County airport. All local economies are different, and these kinds of extrapolations can mask important differences between Orange County and other locations -- the proximity of RDU as an alternative transportation option being a particularly glaring example.

Second, the analysis behind the Talbert & Bright study assumes that any spending by visitors coming to Orange County by way of a new airport represents "new money" injected into the local economy. This is clearly incorrect. Many (probably most) individuals who might like to have flown into a new Orange County airport would instead simply choose an alternative means of traveling -- like flying into RDU or driving. In fairness, there probably would be some individuals choosing not to travel to Orange County unless they could fly into a general aviation airport, but my sense is that that number would be quite small.

Finally, some of the annual economic impacts reported in the Talbert & Bright study are so large as to raise huge red flags about the survey-based data that forms the basis for computing the share of businesses' revenues attributable to their local general aviation airports. For example, the relatively small Sanford-Lee airport is listed as generating over $280 million in annual economic benefits -- more than a quarter of total annual earnings in the county! Numbers such as these suggest serious methodological shortcomings that call into question claims of a new Orange County airport serving as an "economic development engine."

There may well be substantial public benefits to a new Orange County airport, such as enhancing the effectiveness of the primary beneficiary of the proposed airport (the North Carolina Area Health Education Centers Program) or providing a replacement for Horace Williams to serve the needs of local private pilots. But as of now, there is no compelling evidence that significantly increased local economic activity will be among those benefits.

Mitch Renkow is a professor of agricultural and resource economics at N.C. State University.