The left-hand panel shows an H₂ excitation diagram for HH 26-IRS. The data, corrected for extinction (A_v = 27 in this case), have been fitted with a straight line and a second-order polynomial (dashed line). The equations describing each fit are displayed; the R² values represent the square of the correlation coefficient associated with each fit. Plots over a range of A_v values were generated and R² measured for the two fits in each case. The right-hand panel shows a plot of R² against A_v. Data from the linear and polynomial fits are represented by a cross and an open square, respectively; the full and dashed lines represent third-order polynomial fits to these data. The peak in each curve in the right-hand plot marks the extinction associated with the greatest value of R², which in turn is associated with the least scatter about the linear and polynomial fits in the excitation diagram.