Already Tycho Brahe suspected that Ptolemy adopted his star coordinates from a star catalog by Hipparchus. The aim of this paper is to prove this by a statistical analysis. First, given that Hipparchus used an equatorial coordinate system and that Ptolemy produced an ecliptic-based star catalog, a derivation of ecliptical coordinates from equatorial ones must have taken place. One version, requiring an equivalent of the arc tangent function is dismissed as too complicated, and an alternative solution is presented, one using only known methods of ancient mathematics. Two stochastic models, considering a conversion by calculation as well as by a globe, indicate that the independence of Hipparchus' and Ptolemy's data is unlikely.