Chapter 15—Power
MULTIPLE CHOICE QUESTIONS
15.1 Knowledgeable and unbiased observers have generally noted, and studies have
reliably shown, that psychotherapy is better than no treatment at all for a person
who is seriously depressed. My study failed to find a difference. From this I can
conclude that
15.2+ All other things being equal, one study will have more power than another study
if
15.3 The fact that the popular press can always find studies that failed to obtain the
same findings as others suggests that
15.4 Sometimes different experimenters find different results because of
15.5 In the text the author complained about being asked “how many subjects do I
need.” His point was that this is a more complex question than it seems because
you need to consider
15.6+ When we are worrying about power we are most concerned with
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15.7 A Type II error is the probability of
15.8 The probability of a Type II error is usually denoted by
15.9 Power is most often conceived of as
15.10+ Which of the following is involved in the calculation of power?
15.11 In general, power increases as
15.12+ If we have two experiments in which the groups are equally different, the one
with the larger power will be the one that
15.13 When we calculate the power for a number of different experimental designs, the
calculations depend upon
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15.14+ To increase power, the easiest variable to control in designing an experiment is
usually
15.15 The effect size (
ˆ
d
ˆ
d
) is
15.16+ The effect size is generally denoted by
15.17+ The effect size can best be thought of as
15.18 When it comes to estimating the effect size we can use
15.19+ When people such as Cohen say that a medium size difference has an effect size
of .50, they are saying that we want the power for
15.20 Which of the following represents Cohen’s rough estimates of small, medium,
and large effect sizes?
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15.21+ In calculating power we calculate a statistic called delta (). This statistic is
15.22 When we use the notation “f(N)” we are denoting
15.23 In calculating power we separate the effect size and the sample size. We do this
so that
15.24 The formula
Nd
ˆ
=
allows us to calculate
15.25+ The text suggested that the highest value of power that we are likely to be able to
afford, assuming that we don’t have huge differences between our groups, is
something like
15.26+ If we use the standard approach to solve for the necessary sample size so that we
have power = .75 in a two-sample t test, the value of N that we will obtain from
the tables is
Test Bank
15.27 When we have two independent groups with different numbers of subjects in the
two groups, and we want to calculate the power we have for a given set of values
of and , the N that we will use in our calculations is
15.28 If we have the possibility of different numbers of subjects in each of two groups,
we will maximize our power if
15.29+ If we have two related samples, the power of our test will
15.30 It is easy to forget whether the entry in the table at the back of the book represents
power or 1 – power. (Different tables do it differently). We can easily figure it
out, however. If the entry is power, we would expect
15.31 Is it more meaningful to find a significant difference with a relatively small
sample size or with a relatively large sample size?
15.32 Do you have more power with a one-tailed test or a two-tailed test?
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15.33+ It is important to think about power because
15.34+ What is the power in an experiment with two independent groups when the null
hypothesis is true?
15.35+ A harmonic mean can be used to calculate the mean sample size for unequal
sample sizes. For _______ the values of k would equal _______.
15.36 Before running an experiment, we decide to calculate power. We find that the
power equals .95. What conclusion can be made?
15.37 Which of the following is a reason why we may NOT find a significant difference
between two groups?
15.38 Suppose a weak relationship exists between fetal alcohol syndrome and
hyperactivity. We are more likely to find that the sample correlation is significant
if
15.39 Power is controlled by
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15.40 Which of the following reduces power?
15.41 Power is defined as the
15.43 What is the easiest way to increase power?
15.44 Which of the following determines how the power of an experiment varies with
sample size?
15.45 You are on an Institutional Animal Care and Use Committee (IACUC) at your
school. You receive a request to run an animal study where the power will be .40.
Do you approve the study?
15.46 When we have two independent samples, the best measure of effect size
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15.47 In computing effect sizes for a comparison of independent samples, our measure
may be less than ideal if
15.48 When we have two independent samples, a confidence limit is generally used to
15.49 We are trying to evaluate the efficacy of a treatment for claustrophobia. We have
a control group that receives no treatment and an experimental group that receives
our new treatment. We ask each participant to enter a very small room and stay
there are long as they can. The mean of the control group is 10 seconds, with a
standard deviation of 4 seconds. The mean of the experimental group is 18
seconds with a standard deviation of 8 seconds. Our best estimate of effect size is
15.50 Suppose that in the previous question the 95% confidence interval on the
difference between group means was computed to be 2.63 ≤ – ≤ 13.37.
What can we conclude about the null hypothesis of no treatment effect?
TRUE/FALSE QUESTIONS
the sample size.
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hypothesis.
identify important differences.
sizes differ substantially.
OPEN-ENDED QUESTIONS
15.61 Identify and explain three factors that affect power.
15.62 Given the following values for , what is power assuming = .05?
a) = 1.55
b) = 3.60
c) = 2.65
15.63 Calculate effect size given the following information:
a)
,30
1=
,35
2=
= 3
b)
,40
1=
= 10
c)
,100
1=
,98
2=
= 5
15.64 A pediatrician is interested in determining if cognitive development scores are
lower for low-birthweight infants than normal infants. She has 49 active cases
involving low-birthweight infants from who she could collect data to compare to
the national average. What would power be assuming the following effect sizes
and = .05?
a)
ˆ
d
=.20
b)
ˆ
d
=.50
c)
ˆ
d
=.80
15.65 A clinician is interested in examining the effectiveness of a new treatment
approach for depression. Currently, 20 people are in the control group and 20
people are in the intervention group. In similar research, clinicians have reported
post-treatment depression scores of 30 and 26 for the control groups and
intervention groups respectively, and a standard deviation of 4.5. What is a
reasonable estimate of power if = .05?
15.66 The effect size for an independent samples t-test is .90. How many subjects
would be necessary for the following levels of power if = .05?
a) .50
b) .77
c) .91
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15.67 The effect size for a one-sample t-test is .45. How many subjects would be
necessary for the following levels of power if = .05?
a) .40
b) .80
c) .95
15.68 In a related samples t-test, husbands and wives scores are correlated such that r =
.55, and the typical standard deviation on the measure of marital satisfaction is 5.
Assuming the true mean difference in marital satisfaction is 5 points, the sample
consists of 10 couples, and = .05:
a) What is
ˆ
d
?
b) What is ?
c) What is power?
15.69 A student preparing to use a one-sample t-test indicated that power = .80 with a
sample of 30. What was the effect size assuming = .05?
15.70 An English teacher believes his method of teaching grammar, usage, and
mechanics is superior to most other approaches. He plans to compare his
students’ average score on a standardized test to the national average. Assuming
a medium effect size, 20 students in his class, and = .05, what is power?
Answers to Open-ended Questions
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Test Bank