Look at that carefully: 55.5% of the American premieres during the 2006-2007 season were produced in theatres with the smallest budgets. In fact, a single Category Six theatre produced nearly as many American premieres all of the theatres in Categories 1 through 5 combined: The Actors Theatre of Louisville (10 productions). Without the Actors Theatre of Louisville's commitment to the American playwright, there would have been a anemic 17 world premiere productions of American plays on LORT stages.
Scott has done an excellent job crunching the numbers, but there is another step that needs to be looked at. How many 2nd or third productions of new American plays were done at these theaters? In other words, there might be a regional theater that did not produce a World Premiere of an American play, but that theater may have produced two or three relatively new American plays. (For instance, new plays by established or emerging playwrights that were premiered by another Lort house a season before or so.) Not that this would take away from his point, but 2nd or 3rd productions can support playwrights as much a world premiere does.