In some fields of history the quantity of evidence is so overwhelming that it would take an unacceptably long time to assess every item fully. Many of the subjects treated in this book fall into this category: for example a full evaluation of every household in the 1851 census returns for a large town would involve an immense amount of work. The usual strategy adopted in the face of an unmanageable quantity of evidence is a narrowing of the scope of the enquiry, in time, in space, or in subject matter. In many cases this may be an acceptable solution, but there are occasions when it is an unwelcome one. For example, interest may be precisely in the development of a large area over a long span of time, or the evidence may be quite manageable except for one vital series of documents of gargantuan size which alone would swallow up the time available for research.

There is, however, an alternative strategy open to us. If we can discover all we need to know by looking at only part of the evidence, that is by sampling it, we can radically reduce the time required to complete the research. As a result we may be able to embark upon research projects which would otherwise have been out of the question, or we may be able to extend either the range of our research by considering further evidence, or its depth by reflecting longer on the significance of our results.