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.
Why do these things only come around once every 17 years? Not all cicadas have a 17-year life cycle. Some, dubbed annual cicadas, have life spans of anywhere from two to eight years, and some members of these species end up emerging every year, so they're not as noticeable. The cicadas we'll see this year, however, are periodical cicadas. Some species have a 17-year life cycle, while others are on a 13-year cycle.