Factor models were fit to provide a formal test of changes in the cross-drug correlations. Model fit in the full male sample was very good (χ2=216.66, df=170, p=.009; ΔAkaike Information Criterion = 123.33; Root Mean Square Error of Approximation = .02). Model fit in the full female sample was also good (χ2=432.32, df=170, p<.001; ΔAkaike Information Criterion = −92.32; Root Mean Square Error of Approximation = .05). Model fit was good in both the male (χ2=137.45, df=170, p=.97; ΔAkaike Information Criterion = 202.55; Root Mean Square Error of Approximation < .01) and female (χ2=133.42, df=106, p=.04; ΔAkaike Information Criterion = 78.6; Root Mean Square Error of Approximation = .04) early-use subsamples.
