The last lecture dealt with averages, measures of 'central tendency', and considered some aspects of statistical notation. Statistical notation is a valuable shorthand for expressing statistical concepts in a formal and rigorous way and is used by all the textbooks and computer based statistics packages. This lecture provides and introduction to variability.
Variability is the degree to which scores in a distribution are spread out.
Variability is an important descriptor of data and it is always difficult to interpret differences in the mean without considering variability. The following example illustrates the point. Lombroso measured the cranial capacities of criminals and of respectable citizens. He found that the mean from 121 male criminals was 1,450cc while the mean from 328 decent citizens was 1,484cc. The frequency distribution of cranial capacities is graphed below.