ABSTRACT
When data are obtained, they can frequently be considered to be a sample (i.e. a few members) drawn at random from a large population (i.e. a set having many members). If the sample number is large, it is theoretically possible to choose class intervals which are very small, but which still have a number of members falling within each class. A frequency polygon of this data then has a large number of small
line segments and approximates to a continuous curve. Such a curve is called a frequency or a distribution curve. An extremely important symmetrical distribution curve is called the normal curve and is as shown in Figure 89.1. This curve can be described by amathematical equation and is the basis of much of the work done in more advanced statistics. Many natural occurrences such as the heights or weights of a group of people, the sizes of components produced by a particular machine and the life length of certain components approximate to a normal distribution.
