Homework 3 Solutions

Ch. 9:

4.  Somewhat higher; see exercise set B, pp. 145.

10.  (a) True.  The test-takers are a self-selected group, and good
students are more likely to take the test.  Hence, if a larger
percentage of high school graduates take the test, the average skill
level goes down.  

     (b) False.  The average in New York is lower - but the reason may
be that in New York, a higher percentage of students take the test.



Ch. 10: 

3.  (a) 64in

  The husband is 4in, or 4/2.7, which is about 1.5 SDs above average
in height.  The wife is predicted to be above average in height by
r x 1.5 = 0.25 x 1.5, which is about 0.4 SDs.  This is 0.4 x 2.5 = 1in.

     (b) 62in

     (c) 63in, the average

     (d) same as (c)
 

4.  (a) 15 years

    (b) 13.5 years

    (c) Appearances are deceiving; all that is going on here is the
regression effect (section 5).


5.  (a) False (unless there is something special about the SDs and the
point of averages)

    (b) False: r measures association, not causation

    (c) True

    (d) True: the correlation between y and x equals the correlation
between x and y.

    (e) False: r measures assocation, not causation.