CLASS 4: Probability
- Coin Tosses
- In a study on how people perceive probability, Kahneman and
Tversky asked subjects ``which of the following two sequences is
more likely: H T H T T H or H H H T T T ? " Most people will say
the first sequence. Why do you think they say this?
- If a fair coin is tossed three times there will be 0, 1, 2, or 3>
heads. How likely is each of these four possibilities?
- Beyond a reasonable doubt
A judge's charge to a jury might by like the following issued by Judge
Weinstein:
``If you entertain a reasonable doubt as to any fact or element
necessary to constitute the defendant's guilt, it is your duty to give
him the benefit of that doubt and return a verdict of not guilty. Even
where the evidence demonstrates a probability of guilt, if it does not
establish such guilt beyond a reasonable doubt, you must acquit the
accused. This doubt, however, must be a reasonable one; that is one
that is founded upon a real tangible substantial basis and not upon
mere caprice and conjecture. It must be such doubt as would give rise
to a grave uncertainty, raised in your mind by reasons of the
unsatisfactory character of the evidence or lack thereof. A reasonable
doubt is not a mere possible doubt. It is an actual substantial doubt.
It is a doubt that a reasonable man can seriously entertain. What is
required is not an absolute or mathematical certainty, but a moral
certainty." State v. Cage, 554 So.2d 39, 41 (La. 1989)
Discussion
- Do you feel a moral certainty that if you roll three dice, you
will not roll three 6's?
- Does it make sense to assign an actual probability to the notion
of a reasonable doubt? If so, what probability would you assign?
- Do you suppose that juries are ever really told what probability
to associate to the phrase `reasonable doubt'?
- What percent of the people on death row do you think are
innocent?
- In an article about the trial of John Bertsch and Jeffrey Hronis
accused of a 1985 kidnap, rape and murder case we read:
``The FBI's DNA tests in 1989 showed that the chances of a match
were 1 in 12 million for Bertsch and 1 in 8 million for Hronis. In
a re-testing in 1992 the FBI came up with 1 in 16,000 for Hronis>
and 1 in 200 for Bertsch."
Would the 1989 tests have satisfied ``beyond a reasonable doubt"?
What about the 1992 tests?
Journal Assignment
- Read Chapter 13, 14, 15 in the textbook. Do the following
problems to be handed in with your journal on Oct. 4th: P221,
#3,4,6,7,11; P235, #1,2,8,9,10; P243, #5,6,7.
- If you haven't done so, Perform experiments with coin tosses, do
it in three ways: flip a coin, balance a coin on edge and strike>
the table, spin a coin on edge. Do each trial 100 times, and
record the number of heads you get.