Probability
is not fixed at the time of an event such as a fingerprint match being found or
a lottery win.
If
I approach a National Lottery player at random, I can be very very sure that
they have not won the Jackpot. If they
tell me otherwise, I can confidently accuse them of lying. This is because the odds of winning the
lottery are 1 in 14 million and I have chosen a player AT RANDOM. If I am sitting next to someone on a train
who tells me that they have won the lottery I would not dispute it because they
came to my attention by a non-random process.
If I gatecrash a lottery winners’ party, the probability of choosing a
lottery winner from among the guests is very high. So we have three different
probabilities all based around the same lottery win.
The
match between fingerprint Y7 and Shirley McKie was brought to the attention of
the prosecutor, and then to our attention, by a non-random process.
NOTE: I think that the points
“Probability varies after the event”, Texas Sharpshooter Fallacy and
Prosecutor’s Fallacy are actually the same logical point expressed in three
different ways..
Stephen
Horn
26
October 2006