Définition
When a population is divided into sub-populations, a figure may evolve in one direction for each sub-population and in the opposite direction for the population as a whole. This paradox can be explained because the numbers of certain sub-populations increase whereas others regress: that is the structuring effect.
For example, the salary of each profession may stagnate (or increase slightly) whereas the average salary increases sharply; this occurs if highly qualified professions, which are paid the most, are more and more numerous and, conversely, if unqualified jobs, which are paid the least, are more and more rare.
Conversely, the structure-constant variation is calculated as a weighted mean of the variations in the means of each sub-population, with the weightings being the weight of magnitude for each sub-population.