Using recent data obtained for decorrelation frequency fν for 20 pulsars with dispersion measure DM ≤ 50 pc cm-3, we investigate whether there is any systematic variation of the scattering index parameter, (<∆N2e>/L)1/2 = ∊, in the interstellar medium (ISM). It is found that ∊ appears to vary uniformly in certain regions of the ISM, defined approximately by 110 ≤ Z < 420 pc and with corresponding Galactic coordinates given by 26° < |b| < 87°. Elsewhere, ∊ is found to have a very random variation. The results for ∊ arising from values of fν obtained earlier by other authors are compared with our result. It is observed that more data are required before a definite conclusion on the mode of variation of ∊ can be made.