Abstract
Is the measurement in clinics of tests based on performance, when only one measurement is taken, a correct way to indicate the true measure of that individual's ability? In this theoretical article we show that this is a wrong way. We first recall Molenaar’s approach when he invokes the ergodic theorems to show that only in ergodic populations it is possible to transpose the population parameters to measure the individuals. Since the individual processes which develops in time are non-ergodic by their own nature, the first author developed the concept of Ergodic Moment and we show that in tests based on performance, a vocabulary test in our example, we may create an ergodic population, which allows us to transpose the population ability to measure the ability of the individual.