In prodiction, the explanandum describes an event posterior to at least one of the particular circumstance sentences (I to In) of the explanans. When the explanandum describes an event temporally antecedent to any events given by particular so-called circumstance sentences in the explanans, then we are dealing with retrodiction. Longitudinal (prospective) research, in which the aim is to account for the appearance of new abilities after some particular circumstance has occurred should, in principle, consist of explanations based on prodictive arguments. In cross-sectional, age-based, comparisons, a new ability is assumed to have occurred before (or with) a particular circumstance and therefore can only really resort to explanations in terms of retrodictive arguments. A lack of appreciation of the differences between these two sorts of explanatory arguments can lead to confusion about when longitudinal or cross-sectional research is most appropriate for a particular developmental question.
See Cross-sectional design, Explanans and explanandum, Explanation, Longitudinal design, Longitudinal studies, Prediction and explanation, Prognosis