The results of tests on man-made materials, such as concrete and steel, often follow a normal (a.k.a. ‘Gaussian’) probability density function (PDF), as shown in Figure 2.15. The normal distribution arises when a physical property depends on the combination of a large number of individual, random effects.7 It is encountered frequently in nature and is one of the most important PDFs in the field of statistics. The horizontal axis of Figure 2.15 measures deviation of the variable X from its mean value and the vertical axis gives the probability density of X.