The paper presents an attempt to assess how random errors and systematic errors in gravity data affect the quality of the geoid model when it is computed using the FFf technique. Three groups of numerical tests were conducted with the use of gravity anomalies for Poland on 2' x 2' and 5' x 5' grid and with simulating random and systematic errors. In the first test, the effect of random errors on calculated geoid undulations was investigated, in the second one - the effect of systematic errors, and in the last one - the combined effect of both random and systematic errors. The effect of density of data set on the propagated error in geoid height was also examined. The results of numerical tests made possible to evaluate the effect of random errors as well as systematic errors on the accuracy of computed geoid undulation. They were also useful in evaluating the quality of the gravimetric quasigeoid model for Poland.