ABSTRACT
The frequency polygon is a variant of the histogram constructed by marking where the mid-points of the tops of the histogram boxes would be and joining these points with straight lines. The pictorial representation of cumulative frequencies is sometimes useful. The frequency polygon for sample measurements of a continuous random variable can be thought of as a single realization from the infinite number of possible polygons. The chapter shows a so-called normal curve, a pattern that is particularly important, partly because most biological variables follow this distribution. It provides an exponential distribution – the pattern appropriate to intervals between events that are occurring at random. The chapter exhibits a lognormal distribution. The variation in income in a country is often of this form, relatively few people having very high incomes; the pattern applies similarly to many other aspects of economic ‘size’. The chapter shows an extreme value distribution.
