Data and information gathered from records and documents must be carefully evaluated so as to attest their worth for the purpose of the particular study. Evaluation of historical data and information is often referred to historical criticism and the reliable data yielded by the process are known as historical evidence. Historical evidence has thus been described as that body of validated facts and information which can be accepted as trustworthy. Historical criticism is usually undertaken in two stages: first, the authenticity of the source is appraised; and second, the accuracy or worth of the data is evaluated. These two processes are known as external and internal criticism respectively.
External criticism is concerned with establishing the authenticity or genuineness of data. It authenticates the document (or other source) itself rather than the information it contains. It therefore sets out to uncover frauds, forgeries, hoaxes, inventions or distortions.
After the document authenticity has been established, the next task is to evaluate the accuracy and worth of the data contained therein. This presents a more difficult problem than external criticism does. The credibility of the author of the documents has to be established. A number of factors must be taken into account, that is 1) whether they were trained observers of the events, 2) kinds of their relationships to the events, 3) to what extent they were under pressure, from fear or vanity, to distort or omit facts, 4) what the intents of the authors of the documents were, 5) to what extent they were experts at recording those particular events, 6) they were too antagonistic or too sympathetic to give true picture, 7) how long after the event they recorded their account, and 8) whether they are in agreement with other independent witnesses.
A particular problem that arises from these questions is that of bias. There are three generally recognized sources of bias: those arising from the subject being interviewed, those arising from themselves as researchers and those arising from the subject-researcher interaction (Travers, 1969).