In the published version of the paper, some typos were introduced (which were not in the manuscript version). Namely, on p. 615 in the formula for the correction factor rho for the corrected chi-square statistic and on p. 616 in the noncentrality parameter in equation (11), in the denominators of these formulae, both N_c^2's should be N_c's. The N_c^2 in the numerators is correct. The results in the paper are not affected by the typos. Thanks to Tim Thornton for identifying these typos.