Even official reports cannot be trusted without scrutiny.

The fact is that many things conspire to make an official report constrained and formal.  There is the natural desire of every man to put the best face on things for himself as he sets his case before the government and the world; subordinates must be let off leniently; you must live with them, and it impairs comfort to have them sullen.  To make a statement unpleasant to a superior might be construed as insubordination.  The public welfare makes it imperative to tell a flattering tale.  The temptation is constant to tell not quite the whole truth, and nothing but the truth.  There are important suppressions of fact in the official records, none more so, perhaps, than as regards Chancellorsville.[17]

If you happen to be dealing with a historical matter, where the testimony comes from a more or less remote past, and the evidence is scrappy and defective, you must be still more careful.

The great English historian, the late S.R.  Gardiner, in his examination of the evidence on the Gunpowder Plot of 1605, wrote as follows about the difficulties of dealing with historical evidence: 

It seems strange to find a writer so regardless of what is, in these days, considered the first canon of historical inquiry, that evidence worth having must be almost entirely the evidence of contemporaries who are in a position to know something about that which they assert.  It is true that this canon must not be received pedantically.  Tradition is worth something, at all events when it is not too far removed from its source.  If a man whose character for truthfulness stands high, tells me that his father, also believed to be truthful, seriously informed him that he had seen a certain thing happen, I should be much more likely to believe that it was so than if a person, whom I knew to be untruthful, informed me that he had himself witnessed something at the present day.  The historian is not bound, as the lawyer is, to reject hearsay evidence, because it is his business to ascertain the truth of individual assertions, whilst the lawyer has to think of the bearing of the evidence not merely on the case of the prisoner in the dock, but on an unrestricted number of possible prisoners, many of whom would be unjustly condemned if hearsay evidence were admitted.  The historian is, however, bound to remember that evidence grows weaker with each link of the chain.  The injunction, “Always leave a story better than you found it,” is in accordance with the facts of human nature.  Each reporter inevitably accentuates the side of the narrative which strikes his fancy, and drops some other part which interests him less.  The rule laid down by the late Mr. Spedding, “When a thing is asserted as a fact, always ask who first reported it, and what means he had of knowing the truth,” is an admirable corrective of loose traditional stories.