ABSTRACT
At the end of this chapter, you should be able to:
• recognise a normal curve • use the normal distribution in calculations • test for a normal distribution using probability paper
When data is obtained, it 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 Fig. 41.1. This curve can be described by a mathematical 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.
