Extra Information!

Core principles
The following core principles of source criticism were formulated by two Scandinavian historians, Olden-Jørgensen (1998) and Thurén (1997):[1]

• Human sources may be relics such as a fingerprint; or narratives such as a statement or a letter. Relics are more credible sources than narratives.
• Any given source may be forged or corrupted. Strong indications of the originality of the source increase its reliability.
• The closer a source is to the event which it purports to describe, the more one can trust it to give an accurate historical description of what actually happened.
• A primary source is more reliable than a secondary source which is more reliable than a tertiary source*, and so on.
• If a number of independent sources contain the same message, the credibility of the message is strongly increased.
• The tendency of a source is its motivation for providing some kind of bias. Tendencies should be minimized or supplemented with opposite motivations.
• If it can be demonstrated that the witness or source has no direct interest in creating bias then the credibility of the message is increased.

Procedures
Bernheim (1889) and Langlois & Seignobos (1898) proposed a seven-step procedure for source criticism in history:[2]

1. If the sources all agree about an event, historians can consider the event proved.
2. However, even if most sources relate events in one way, that version will not prevail unless it passes the test of critical textual analysis.
3. The source whose account can be confirmed by reference to outside authorities in some of its parts can be trusted in its entirety if it is impossible to confirm the entire text.
4. When two sources disagree on a particular point, the historian will prefer the source with most "authority"—that is the source created by the expert or by the eyewitness.
5. Eyewitnesses are, in general, to be preferred especially in circumstances where the ordinary observer could have accurately reported what transpired and, more specifically, when they deal with facts known by most contemporaries.
6. If two independently created sources agree on a matter, the reliability of each is measurably enhanced.
7. When two sources disagree and there is no other means of evaluation, then historians take the source which seems to accord best with common sense. 

        * Tertiary Source - A term used to describe a work chiefly a selection or compilation of other primary or secondary                         
                                               sources.