ABSTRACT
In this part we will continue to work with a set of data which we will denote by {x1, x2, …, xN}. Its mean is denoted by and its standard deviation is denoted by s. In part 2 we contented ourself with studying this set and giving some characteristics of it. However, there is a need for more. Let us show this by giving an example. Suppose the data are on delivery times of books by a bookseller and suppose we find a value of =90 days. Suppose also that, from earlier analogous tests, we know that used to be around 2 months. We then go to this bookseller and tell them that there must be an improvement in the service. Then two things will happen. The bookseller will say first that there is a constant effort to improve services, and then that the tested N (say 50) books are just too small a set in order to state that services have deteriorated. What can we do about this?
