Motility and orientation of motile microorganisms can be quantified basically by two approaches: population methods and individual tracking. Individual tracking analyzes the movement parameters of single organisms and subsequently averages over the behavior of a statistically significant number of individuals to evaluate the behavior of a population. Population methods operate on the assumption that the movement of the individual organisms will lead to a translocation of the whole population. Both methods have their advantages and drawbacks. Individual tracking is tedious and can be prone to error or bias, and may only be applicable for restricted path segments. Reaching a significant conclusion for a whole population may strain the patience of an experimenter. On the other hand, population methods may measure something different than the experimenter assumes. For example, a population may seem to be moving in a certain direction and end up, for instance, at the top of a water column, which may be interpreted as the result of negative gravitactic orientation. However, the behavioral result may be due to the organisms moving in random directions, but become immobile near the water surface. Other phenomena leading to a displacement of a population, erroneously interpreted as the result of gravitaxis, include sedimentation, phobic responses, kinetic effects, or the response to other stimuli — including light, magnetic field lines, or chemical gradients (such as oxygen, carbon dioxide, or nutrients).