Of particular interest is whether the magnitude or direction of the pulsar kick has any correlation with other properties of the pulsar, such as the spin axis, magnetic moment, or magnetic field strength. To date, no correlation has been found between the magnetic field strength and the magnitude of the kick. However, there is some contention over whether a correlation between spin axis and kick direction has been observed. For many years, it was believed that no correlation existed. In studies of the Vela and Crab pulsars, jets have been observed which are believed to align with the spin axis of the pulsar. Since these jets align very closely with the bow shock as well as the directly measured velocity of the pulsars, this is considered strong evidence that these pulsars have kicks aligned with their spin axis. It is also possible to measure the spin axis of a pulsar using the polarization of its radiation, and a recent study of 24 pulsars has found a strong correlation between the polarization and kick direction. Such studies have always been fraught with difficulty, however, since uncertainties associated with the polarization measurement are very large, making correlation studies troublesome.