When we have a set of statistical data grouped in classes, we must organize the data so that all of it is contained in one class (not less, not more). The relative cumulative frequence of any class, is the percentage of the data points that are either in this class or are smaller in value than the lowest number of this class. Hence, when we reach the last class (with the larget values), evaluating its relative cumulative frequency means finding what percentage of the observations has a value that belongs to this class or a class with lower values. There is no class with values larger than the last one, so since all the observations must belong to a class, all of the observation have a value in this class or in a class with lower values. Hence the relative cumulative frequency of the last class is always 1 or 100%
