A question on the accuracy of determining the dispersion measure DM for PSR 0809+74 arises in connection with the discussion which has developed in recent years concerning observations of the effect of the superdispersion time delay (STD) (Shitov 1985, Shitov, Malofeev, and Izvekova 1988, Kuz'min 1985a) attributed to the twisting of the magnetic field lines or to, competing with it as to the observation manifestations, the aberration effect (Cordes 1978, Matese and Whitmire 1980, Ulyanov 1989, Ulyanov 1990). An interpretation of the available data in favor of any of these effects would be inappropriate without a quantitative analysis of the observation errors. With our aim to define the DM value and its errors more precisely, we shall analyze both the observational data available elsewhere and our results for PSR 0809+74 which, from the observational data in Shitov (1985), Shitov et al. (1986), and Kuz'min et al. (1985), has the most pronounced STD effect.