Due to the many close encounters with the inner planets, Near-Earth Asteroids (NEAs) show very chaotic orbits. This can cause major problems when one is dealing with the statistic of NEAs. This paper will investigate the feature of mixing: Because of the chaotic behaviour it is more or less impossible to exactly calculate the motion of an single asteroid for longer time scales. Therefore, one has to deal with groups of asteroids. But the existing classifications of NEAs show a feature called mixing. That means, that asteroids can change their classes, when the integration time is long. We show, how the mixing can affect the existing classifications and causes problems when calculating statistical properties of NEAs.