Introductory Econometrics: A Modern Approach 6th Edition by Jeffrey M Wooldridge

Exercise 5

Just prior to jury selection for O. J. Simpson’s murder trial in 1995, a poll found that about 20% of the adult population believed Simpson was innocent (after much of the physical evidence in the case had been revealed to the public). Ignore the fact that this 20% is an estimate based on a subsample from the population; for illustration, take it as the true percentage of people who thought Simpson was innocent prior to jury selection.

Assume that the 12 jurors were selected randomly and independently from the population (although this turned out not to be true).

(i) Find the probability that the jury had at least one member who believed in Simpson’s innocence prior to jury selection. [Hint: Define the Binomial(12,.20) random variable X to be the number of jurors believing in Simpson’s innocence.]

(ii) Find the probability that the jury had at least two members who believed in Simpson’s innocence. [Hint: P(X ? 2) = 1 - P(X ? 1), and P(X ? 1) = P(X = 0) + P(X = 1).]

Step 1 of 2

The probability that Simpson was innocent is 0.2; the probability of Simpson not being innocent is 0.8.

(i) The probability of at least one jury finding him innocent would be equal to 1 minus no jury finding Simpson innocent. Therefore the probability of at least one juror out of 12 juror finding him innocent would be:

The probability of at least one juror found Simpson innocent would be 0.93128.