is the poll likely to find?
Answer : The poll is likely to find that, overwhelmingly, respondents think the coach should be fired.
This is a voluntary response poll, and we know that such a poll is most likely to draw a response
from those who feel most strongly about the issue being polled. Fans who bother to vote in a call-in
poll such as this are most likely upset at their team’s loss and are looking for someone to blame—this
gives them the opportunity. There is, of course, a chance that the coach may be very popular and draw
support, but the point to remember is that this is a self-selecting nonrandom sample, and will
probably exhibit response bias.
It is known that exercise and diet both influence weight loss. Your task is to conduct a study of the
effects of diet on weight loss. Explain the concept of blocking as it relates to this study.
Answer : If you did a completely randomized design for this study using diet as the treatment variable,
it’s very possible that your results would be confounded by the effects of exercise. Because you are
aware of this, you would like to control for the effects of exercise. Hence, you block by exercise
level. You might define, say, three blocks by level of exercise (very active, active, not very active)
and do a completely randomized study within each of the blocks. Because exercise level is held
constant, you can be confident that differences between treatment and control groups within each
block are attributable to diet, not exercise.
- Explain the concept of a double-blind study and why it is important.
Answer : A study is double-blind if neither the subject of the study nor the researchers are aware of
who is in the treatment group and who is in the control group. This is to control for the well-known
effect of people to (subconsciously) attempt to respond in the way they think they should. - You are interested in studying the effects of preparation programs on SAT performance. Briefly
describe a matched-pairs design and a completely randomized design for this study.
Answer : Matched pairs : Choose, say, 100 students who have not participated in an SAT prep
course. Have them take the SAT. Then have these students take a preparation course and retake the
SAT. Do a statistical analysis of the difference between the pre- and postpreparation scores for each
student. (Note that this design doesn’t deal with the influence of retaking the SAT independent of any
preparation course, which could be a confounding variable.)
Completely randomized design : Select 100 students and randomly assign them to two groups, one
of which takes the SAT with no preparation course and one of which has a preparation course before
taking the SAT. Statistically, compare the average performance of each group.
Practice Problems
Multiple-Choice
Data were collected in 20 cities on the percentage of women in the workforce. Data were collected
in 1990 and again in 1994. Gains, or losses, in this percentage were the measurement upon which the
study’s, conclusions were to be based. What kind of design was this?
I. A matched-pairs design
II. An observational study
III. An experiment using a block design
(a) I only