Econ 57 Fall 2002 Final Exam Answers
2. Correlation is not causation.
Perhaps the factors that lead to smoking, drinking, and so on also lead to body
piercing.
3. Because the sample is
with replacement, the size of the population doesnt matter. Under the
null hypothesis that p1 = p2, the difference in sample proportions is approximately
normally distributed with a mean of 0 and a standard deviation equal to
The probability that
the difference between the number of women in the two samples would be less
than or equal to 1 is
The probability that Z will
be between -0.0598 and 0.0589 is 0.047. 4. One minus the probability
of no fours is 1 - (45/47)(44/46)(43/45) = 0.125.
5. The binomial model gives
us the probability of exactly 5 heads and 5 tails:
6. If each student has a
0.2461 probability of guessing 5 heads and 5 tails (H0: p = 0.2461), then the
probability that 27 or more of 56 students would guess 5 heads and 5 tails is
given by the binomial distribution:
7. If you aim for the upper
box first, there is a 0.5 probability of landing in that box and a 0.5 probability
of landing in the lower box. If the ball lands in the upper box, you will toss
your second ball into the lower box. If the first ball lands in the lower box,
you will try for the upper box again.
Your probability
of winning is 0.5 + 0.5(0.5) = 0.75. If, instead, you aim for the lower box
first, your ball will land in it with certainty. You will then aim for the upper
box and have a 0.5 probability of winning. So, it is better to aim for the upper
box first. 8. This is a sample mean
and it is consequently normally distributed with a mean equal to the actual
time and a standard deviation equal to 10 seconds divided by the square root
of 20, which is 2.24 seconds
9. There are a total of
7936 observations and, under the null hypothesis, the expected value for each
of the 10 digits is 7936/10 = 793.6. The chi-square statistic is
With 10 - 1 = 9 degrees of freedom,
the p value is less than 0.00000.
10. A normal approximation
is
The two-sided p value is 2(0.318)
= 0.636. The exact p value is given by the binomial distribution: The two-sided p value is 2(0.326)
= 0.652. 11. Using Bayess Rule,
12. Whether the observed
differences are substantial would be gauged by subjectively judging the difference
between the observed and expected values. Since the p value is not below 0.05,
the results are NOT statistically significant at the 5% level.
13. This is true if the average
male and female students were the same age and had fathers of the same height
and mothers of the same height.
14. The probabilities are
The expected value of the
payoff is about an 8 percent loss per dollar wagered: 0(0.5787) + 2(0.3472)
+ 3(0.0694) + 4(0.0046) = 0.921 15. The performance of a
professional football team depends on its ability and also on chanceunpredictable
variations in the opponents play calling, the players health, the
officiating, and even the proverbial bounces of the football. When such chance
events make observed performance an imperfect measure of ability, observed differences
in performance tend to exaggerate underlying differences in abilities. Performances
consequently regress toward the mean, in that those teams that perform the best
at any point in time typically do not perform as well subsequently, and those
teams that perform the worst at any point in time typically do not perform as
poorly subsequently. Marcus recommended betting for teams that had been doing
poorly and against teams that had been doing well, and it worked!
16. A simple regression of
the length gain on birth weight is evidently negative because heavier babies
also tend to be taller; the relatively small positive effect of birth weight
on length gain is swamped by the negative effect of birth length on length gain.
In fact, birth length and birth weight are positively correlated (the correlation
coefficient is 0.57).
17. It is not a flaw since
the test statistic takes into account the (possibly unequal) sample sizes.
18. The 0.5721 p value means
that, if the population mean grades were equal for males and females, there
is a 0.5721 probability that the difference in the sample mean grades would
as large as actually observed: 7.500 - 7.125 = 0.375.
19. 75% of the variation
in the prices is explained by the number of pages.
20. There is self-selection
bias here; the people who play Bingo are much older than those who play Mortal
Kombat.