method has been used to analyze sub-samples of spiral galaxies extracted from the Mark III [@velmod][@velmod1] and the SFI data[@branchini], yielding $\beta_I=0.49$ and $\beta_I=0.42$, respectively. These results show that the value of $\beta_I$ obtained from velocity-velocity comparisons is independent not only of the data set considered but also of the method used, with all estimates being in the range $0.4\lsim \beta_I \lsim 0.6$.

Unfortunately, until recently there has been a disparity between the results obtained from velocity-velocity comparisons and other methods such as density-density comparisons and maximum-likelihood estimates of the power-spectrum (PS) of mass fluctuations derived from peculiar velocity data [@zaroubips]. For instance, density-density comparisons using different data sets have invariably led to high values of $\beta$[@sigad], consistent with unity. In particular, comparison of the 1.2 Jy and POTENT reconstructed density field, based on the Mark III catalog, yields $\beta_I=0.89$. Similarly, estimates based on the PS derived from peculiar velocity data using Mark III[@zaroubips], SFI[@freudling] and ENEAR[@wfenear] have yielded values of $\beta$ in the range 0.82-1.1. The nature of this discrepancy is unknown. Both density-density and velocity-velocity methods have been carefully tested using mock catalogs extracted from N-body simulations and have been shown to provide unbiased estimates of $\beta$. Possible 