Definition of populations. The random quantities used within models for a risk assessment process need to be related to a meaningful and consistent set of populations. The description of the random quantities should correspond to this set and the resulting failure probability is only valid for the same set. The basis for the definition of a population is in most cases the physical background of the variable. Factors which may define the population are: the nature and origin of a random quantity the spatial conditions (e.g. the geographical region considered) the temporal conditions (e.g. the intended time of use of the structure considered) The choice of a population is to some extent a free choice of the modeller. It may depend on the objective of the analysis, the amount and nature of the available data and the amount of work that can be afforded. In connection with theoretical treatment of data and with the evaluation of observations, it is often convenient to divide the largest population into sub- populations which in turn are further divided in smaller sub-populations etc. Then it is possible to study and distinguish variability within a population and variability between different populations. In an analysis for a specific structure, it may be efficient to define a population as small as possible as far as use, shape and location of the structure are concerned (this is referred to as micro-zonation). When the results are used for design on a larger scale (for example in a national or international code), it may be necessary or convenient to put the sub-populations together to the large population again in order not to get too complicated rules, meaning that the variability within the population is now increased.
