The first two of these refer to the variability of loads and material properties. Such variabilities can often be expressed in terms of probability distributions. However, when it comes to the extreme ends of the distributions, lack of statistical information can make precise probabilistic analysis impossible. Let us consider the variability of the properties of materials. Experimental data on material properties are often insufficient for making a distinction between, e.g., gamma and lognormal distributions, a problem called distribution arbitrariness . This has little effect on the central part of these distributions, but in the distribution tails the differences can become very large. This is a major reason why safety factors are often used for design guidance instead of probabilities, although the purpose is to protect against failure types that one would, theoretically, prefer to analyse in probabilistic terms. As Zhu puts it: