Abstract. The correlation between Rate of TEC index (ROTI) and amplitude scintillation index S4 has been under focus for years, since both of the two are associated with ionospheric irregularities. Previous studies show that the behavior of the correlation between ROTI and S4 is not regular. To address this problem, in the present work the relation between S4, ROTI and the Differential Rate of TEC index (DROTI) has been investigated, assuming the single screening model for trans-ionospheric radio propagation. The influences of the effective velocity and elevation angle and their variability on the correlation between ROTI and S4 are analyzed. Data from two stations of Scintillation Network Decision Aid (SCINDA) network are used for the study. Results show that the variability of effective velocity plays a crucial role on the correlation between the amplitude scintillation index and the TEC rate of change indices. Based on the Gaussian assumption of effective velocity distribution, a high correlation coefficient can be achieved only under low variability conditions.