The sum of the absolute frequencies is equal to the total number of data, represented bas $$N$$. 1  Relative frequency: the result of dividing the absolute frequency of a certain value by the total number of data.  The sum of the relative frequencies is equal to $$1$$. We can prove this easily by factorizing $$N$$.
Periodical cicadas emerge from underground every 13 or 17 years, depending upon which brood the cicadas belong to. The year a particular brood emerges often differs from other broods and is also dependent on geography. The periodical cicada, although unrelated, is often mistakenly called a locust.
Cicadas spend years underground and come out once in a blue moon for a frenzy of activity. But certain broods, like the one that emerged in 2004, come out like clockwork every 13 or 17 years. In this Science Update, you'll hear what's so special about these numbers. Why cicadas come out in their prime.