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Bound_up comments on Absence of Evidence Is Evidence of Absence - Less Wrong
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Frank: It is impossible for A and ~A to both be evidence for B. If a lack of sabotage is evidence for a fifth column, then an actual sabotage event must be evidence *against* a fifth column. Obviously, had there been an actual instance of sabotage, nobody would have thought that way- they would have used the sabotage as more "evidence" for keeping the Japanese locked up. It's the Salem witch trials, only in a more modern form- if the woman/Japanese has committed crimes, this is obviously evidence for "guilty"; if they are innocent of any wrongdoing, this too is a proof, for criminals like to appear especially virtuous to gain sympathy.
I think you're right, but there's an adjustment (an update, isn't it called?) warranted in two directions.
The absence of sabotage decreases the likelihood of the fifth column existing at all.
But if there is a fifth column, it could be reasonably predicted that there would be evidence of sabotage unless there was an attempt to keep a low profile. If they were to favor this hypothesis for other reasons, as in the classified data mentioned by Frank, then the lack of apparent sabotage would also increase the probability that if the unlikely fifth column DID exist, it would be one which is keeping a low profile. I grant, of course, at the same time, the decreased probability of there being any kind of fifth column in the first place.