chapter  18
26 Pages

Statistics and Statistical Process Control (SPC)

The Memory Jogger [2] is a useful pocketbook detailing quality control tools together with management planning tools. Statistics can be regarded as the collection, representation and interpretation of data [1]. In a set of data, it is convenient but not necessary, to arrange the data from smallest to largest (e.g. Table 18.1). It is normal to sample 50-100 data points and to compress this information, the data can be divided into a number of equal class intervals, or groups. The data in Table 18.1 has been grouped into ten classes and the frequency recorded in Table 18.2. If the data has not been sorted, then it is helpful to prepare a tally chart (Table 18.2), by recording a tally mark against the appropriate class for each observation. The tallies are then totaled to obtain the respective frequencies. It is the convention to record the tallies in sets of five with the fifth denoted as a diagonal line through the previous four.