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.
How many species are there? There are about 3,000 known species of cicada worldwide, according to the Smithsonian Natural Museum of History. There are annual cicadas, which appear every year, and periodical cicadas, such as the ones emerging in the northeast U.S., which appear in 13- and 17-year life cycles.