brazil84 stated that there are just two options, so let’s stick to that example first.
“[rifle] no bullet will be find in or around the person’s body 0.01% of the time” is contradictory evidence against the rifle (and for the handgun). But “[handgun] no bullet will be find in or around the person’s body 0.001% of the time” is even stronger evidence against the handgun (and for the rifle). In total, we have some evidence for the rifle.
Now let’s add a .001%-probability that it was not a gunshot wound—in this case, the probability to find no bullet is (close to) 100%. Rifle gets an initial probability of 60% and handgun gets 40% (+ rounding error).
So let’s update:
No gunshot: 0.001 → 0.001
Rifle: 60 → 0.006
Handgun: 40 → 0.0004
Of course, the probability that one of those 3 happened has to be 1 (counting all guns as “handgun” or “rifle”), so let’s convert that back to probabilities:
0.001+0.006+0.0004 = 0.0074
No gunshot: 0.001/0.0074=13.5%
Rifle: 0.006/0.0074=81.1%
Handgun: 0.0004/0.0074=5.4%
The rifle and handgun numbers increased the probability of a rifle shot, as the probability for “no gunshot” was very small. All numbers are our estimates, of course.
brazil84 stated that there are just two options, so let’s stick to that example first.
“[rifle] no bullet will be find in or around the person’s body 0.01% of the time” is contradictory evidence against the rifle (and for the handgun). But “[handgun] no bullet will be find in or around the person’s body 0.001% of the time” is even stronger evidence against the handgun (and for the rifle). In total, we have some evidence for the rifle.
Now let’s add a .001%-probability that it was not a gunshot wound—in this case, the probability to find no bullet is (close to) 100%. Rifle gets an initial probability of 60% and handgun gets 40% (+ rounding error).
So let’s update: No gunshot: 0.001 → 0.001 Rifle: 60 → 0.006 Handgun: 40 → 0.0004
Of course, the probability that one of those 3 happened has to be 1 (counting all guns as “handgun” or “rifle”), so let’s convert that back to probabilities: 0.001+0.006+0.0004 = 0.0074 No gunshot: 0.001/0.0074=13.5% Rifle: 0.006/0.0074=81.1% Handgun: 0.0004/0.0074=5.4%
The rifle and handgun numbers increased the probability of a rifle shot, as the probability for “no gunshot” was very small. All numbers are our estimates, of course.