Since psychology emerged as an independent field of knowledge, there has been no consensus as to how it should develop, either, in the idiographic or nomothetic way. In the course of time, due to a commitment to what was seen as objectivity in science, the nomothetic approach came to dominate psychology. Thus, researchers used mostly quantitative psychometric methods to establish general rules of human behaviour. In doing so, the essence of nomothetic research is to be extremely careful when interpreting results not to make a reasoning mistake such as the ecological fallacy, as may happen when a researcher draws conclusions about nature of the individual in the group based on average results of the whole group. In the article, we presented two methods for longitudinal research designs which address this problem, and give more idiographic information about participants; via the Reliable Change Index and the Modified Brinley Plot. Finally, we provide a IBM SPSS Statistics syntax automatizing the whole process of computation for these new features.