I first heard the term "fair assumptions" during a talk given by Susan Campbell Bartoletti and Elizabeth Partridge. The example given involved a scene in which a gas lamp hissed. "How do you know the gas lamp hissed?" a student asked. Elizabeth explained that the details of that scene were taken from a photograph that showed a gas lamp hanging in the background. Elizabeth had used that kind of gas lamp and she'd heard it hiss. She made a fair assumption that all gas lamps hiss therefore the lamp in the photo hissed and the people in the photo would have heard it, even though nowhere in any written historic document had anyone mentioned the noise the gas lamp made.
Last week I mentioned Erik Larson's book Isaac's Storm, and he, too, makes a few fair assumptions which he notes in the back matter. Some are based on photographs. With a magnifying glass Larson picked out specific items like a hat, clothing, etc,strewn in the debris that Isaac would probably have seen although he never wrote about them in any letter or journal.
Another refers to his description of Isaac's family going to either the Murdoch's bath house or the Pagoda bath house on Sunday. Larson admits that he found no documentation proving this, but asserts that the close proximity, and the "communal character of the time -- and the absence of television -- it is all but certain that the Clines did so."
Another example is Larson's assumption that venomous snakes competed with people for space in trees to escape the flood and bit people who then may have fallen and drowned. No one knows if this really happened in Galveston, but the phenomenon has been reported elsewhere. Snakes most certainly would have crawled to higher ground, but would they have bitten their competitors? I know I would out of fear and survival instincts.
The most understandable assumption involves dreams. As Larson says, "I base this observation on human nature. What survivor of a tragedy has never dreamed that the outcome had been different."
Would you make the same assumptions? Every writer is different.
Before you include a fair assumption in your own text ask yourself: Does it change the story? Does it change the reader's perception of the event? If so, don't do it.
This tool should also be used judiciously. In Larson's 300 page book, which includes 15 pages of notes, I found only 7 instances where he had to explain his use of fair assumption.
And like Larson, explain your reasoning in the back matter. Don't let your reader assume you made anything up.