Optical depth ratio (R_67/65 = τ₁₆₆₇/τ₁₆₆₅) as a function of excitation temperature difference (T_ex(1667) − T_ex(1665)) for the 1667 and 1665 MHz OH lines. The horizontal and vertical dashed lines indicate 1.8 and 0.0, the values for LTE excitation. The error bars indicate the 1σ formal uncertainties propagated through from the Gaussian fits. The black points are those consistent with LTE to within the 1σ errors; the red points are inconsistent. The vertical shaded region represents ﹩| {\rm{\Delta }}{T}_{\mathrm{ex}}| \lt 2\,{\rm{K}}﹩.