Reasoning about probability

Trial by Media

There are many ways to approach the thorny issue of “reasonable doubt”.

Jury instructions may give guidance in this way:

“There are very few things in this world that we know with absolute certainty, and in criminal cases the law does not require proof that overcomes every doubt.”

Estimates suggest that about 5% of people convicted and sentenced to death are innocent, meaning juries are regularly making mistakes, thinking a case for guilt has been proved beyond a reasonable doubt when this is not the case.

The reasons for these errors are undoubtedly complex and varied, but here I want to concentrate on fallacious reasoning about probability. Sometimes people talk about “too many coincidences”, when I hear that claim I immediately suspect fallacious reasoning.

How should a jury deliberate? Well, it would be good to reduce that error rate from 5% to a more acceptable rate, let’s say 1 in…

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