Specific capacity () data are usually much more abundant than transmissivity () data. Theories which assume uniform transmissivity predict a nearly linear relationship between and . However, linear dependence is seldom observed in field studies. Since hydrogeologic studies usually require data, many hydrogeologists use linear regression analysis of versus data to estimate values where only data are available. In this paper we use numerical models to investigate the effects of aquifer heterogeneity on the relationship between and estimates. The simulations of hydraulic tests in heterogeneous media show that estimates of derived using Jacob's method tend to their late‐time effective value much faster than values. The latter are found to be more dependent upon local transmissivities near the well. This explains why the regression parameters for versus data depend on heterogeneity and the‘lateness’of the test period analyzed. Since this effect is more marked in high zones than in low zones, we conclude that natural aquifer heterogeneity can explain the convex deviation from linearity often observed in the field. A further result is that the geometric mean of estimates, obtained from short and intermediate time pumping tests, seems to systematically underestimate effective () of heterogeneous aquifers. In the studied simulation cases, the median of the values or the arithmetic mean yield better estimates for .