(4) _Sampling_ is a process of enumeration restricted to a few units taken at different points in the field of investigation; we calculate the proportion of cases (say 90 per cent.) where a given characteristic occurs, we assume that the same proportion holds throughout, and if there are several categories we obtain the proportion between them. In history this procedure is applicable to facts of every kind, for the purpose of determining either the proportion between the different forms or usages which occur within a given region or period, or the proportion which obtains, within a heterogeneous group, between members belonging to different classes. This procedure gives us an approximate idea of the frequency of facts and the proportion between the different elements of a society; it can even show what species of facts are most commonly found together, and are therefore probably connected. But in order that the method may be employed correctly it is necessary that the samples should be representative of the whole, and not of a part which might possibly be exceptional in character. They should therefore be chosen at very different points, and under very different conditions, in order that the exceptions may compensate each other. It is not enough to take them at points which are _distant_ from each other; for example, on the different frontiers of a country, for the very circumstance of situation on a frontier is an exceptional condition. Verification may be had by following the methods by which anthropologists obtain averages.