Download 8 Independent and Dependent t

Homework #8: Independent & Dependent T-tests
1. Reviewing z and t-scores: Matilda Matador scores a 30 on the extraversion scale whereas
normal people score 40 (σ=5). What percent of people are more extraverted Matilda?
a. Are you
dealing with a
score or a
sample mean?
Frequency or
sampling
distribution?
c. Roughly sketch the distribution and
value.
b. Find the z or t-score.
x = 30
μ = 40
σ=5
z=?
z
x

.4772
30  40
z
 2 .0
5
.500
score
frequency dist.
d. Find the
correct
percent (for zscores only).
.4772
.5000
.9772
97.72%
more
extraverted
z = -2.0
2. Reviewing z and t-scores: A group of 25 teenagers forced to watch 20 hours of Barney
average 41 on the depression inventory (μ=40, σ=10). What percent of all teenagers are less depressed
than this group?
a. Are you
dealing with a
score or a
sample mean?
Frequency or
sampling
distribution?
sample mean
sampl. distribut.
b. Find the z or t-score.
M = 41
μ = 40
σ = 10
n = 25
z=?
x 
z
z
x
n
c. Roughly sketch the distribution
and value.

10
2
25
x
.1915
.500
x
41  40
 0 .5
2
d. Find the
correct
percent (for zscores only).
z = 0.5
.1915
.5000
.6915
69.15%
less depressed
3. Reviewing z and t-scores: The “Safe and Speedy” moving company told Opal that the
average shipping time was 7 days. Former customers indicated delivery times of 4, 9, 10, 5, 12, and 10
days. Does the 7 days avg. seem plausible based on these data?
a. Are you
dealing with
a score or a
sample
mean?
Frequency
or sampling
distribution?
sˆ x 
sˆ x 
b. Find the z or t-score.
μ=7
M=8.3333
ŝx = 3.1411
n=6
x
sˆ x
c. Roughly sketch the distribution and value.

2
2

 x

n
n 1
3.1411
tobt=1.0398
2500
6  3.1411
6 1
466 

 1.2823
n
6
x   8.3333  7
t

 1.0398
sˆ x
1.2823
tc = 2.571
tc = +2.571
Retain Ho.
7 day average is plausible.
4. Interpreting Independent t-tests: An educational psychologist speculated that students who
spent more time reading would have lower hostility scores because they would be better able to reason
through to problem solving and express their feelings to others. She designed a reading intensive
summer experience that students took each year of junior high school. She randomly 20 students both a
control and experimental condition, and evaluated their hostility scores after 3 years.
Data:
Con
trol
20
30
40
30
20
25
20
30
35
40
Exp
15
10
15
20
10
25
30
20
20
20
Group Statistics
HOSTIL
G
R
O
1
U
2
P
N
10
10
Std.
Deviation
7.746
6.258
Mean
29.00
18.50
Std. Error
Mean
2.449
1.979
Independent Samples Test
Levene's Test for
Equality of Variances
F
HOSTIL
Equal variances
as sumed
Equal variances
not ass umed
t-test for Equality of Means
df
Mean
Differe
nce
Std. Error
Difference
3.33
18
.004
10.50
3.149
3.884
17.116
3.33
17.2
.004
10.50
3.149
3.863
17.137
Sig.
.635
t
.436
95% Confidence
Interval of the
Difference
Lower
Upper
Sig.
(2-ta
iled)
* Label as much of the output as possible
with the correct symbols. Be sure to
distinguish between standard error of the
mean and standard error of the difference.
* Show how you’d set up the data to enter it into SPSS
Group
Hostility
1
20
1
30
…
…
2
15
2
10
a) 18.50 What’s the average level of
hostility in the experimental group?
Hypothesis Testing Steps:
b) 29.00 What’s the avg. level of
hostility in the control group?
c) 10.50 What’s the observed
variability?
d) 3.149 What’s the expected
variability?
e) 3.33 What’s tobt?
f) .4% What’s the probability you’d
see this difference between sample
means by chance?
1. Compare M1 & M2
2. Ho: μ1 – μ2 = 0
Ha: μ1 – μ2 = 0
3.  = .05, df = 18 tcrit = 2.101
4. tobt = 3.33
sˆ  sˆx * n  3.149 * 10  9.9580
d
x1  x2 29  18.5

 1.0544
sˆ
9.9580
5. Reject Ho.
The hypothesis was supported. The average hostility
score for the experimental group (M=18.50), was
significant lower than that of the control group
(M=29.00), t(18) = 3.33, p.05. Reading has a large
effect on hostility scores, d=1.0544.
5. Interpreting Dependent T-tests: An I/O psychologist conducts a study to examine the impact
of a diversity training workshop for managers. He asks subordinates to rate managers both before and
after the weekend workshop to see if managers have become more sensitive (e.g., less likely to use
racial stereotypes, more sensitive to the needs of working mothers, respectful of non-Christian holiday
requests, etc.). The subordinates rate their supervisors using a measure of tolerance developed by the
psychologist. Scores range from 10 (very insensitive) to 50 (extremely sensitive).
xbefore  26.25
Data
xafter  31.88
Paired Samples Statistics
Before
25
20
25
30
25
20
25
40
After
35
20
30
25
30
40
25
50
Pair
1
BEFORE
AFTER
Mean
26.25
31.88
BEFORE - AFTER
* Label as much of the output as possible
with the correct symbols. Be sure to
distinguish between standard error of the
mean and standard error of the difference.
26.25 What’s the average level of
tolerance before?
b) 31.88 What’s the avg. level of tolerance
after the training?
c)
sˆ x  6.409,9.613
sˆ x  2.266,3.399
Std. Error
Mean
2.266
3.399
D  5.63
sˆD  7.763
Paired Samples Test
Pair 1
a)
N
8
8
Std.
Deviation
6.409
9.613
–5.63 What’s the observed difference?
d) 2.745 What’s the expected difference?
e)
–2.049 What’s tobt?
f)
8% What’s the probability you’d see
this difference between sample means
by chance?
Mean
-5.63
Paired Differences
95% Confidence
Interval of the
Std.
Std.
Difference
Deviat
Error
ion
Mean
Lower
Upper
7.763
2.745
-12.12
.87
sˆD  2.745
t obt  2.049
pobt  .080
t
-2.049
df
7
Sig.
(2-tailed)
.080
* Show how you’d set up the data to enter it into SPSS
Before After
25
35
20
20
25
30
…
….
Hypothesis Testing Steps:
1. Compare Dbar & μD
2. Ho: μD = 0 ;
Ha: μD  0
3. α =.05, df = n-1= 7; tcrit= 2.365
4. tobt = -2.049
5. Retain Ho.
The hypothesis was not supported. The average
tolerance level of managers after training (M=31.88) is
not statistically different from the level before training
(M=26.25), t(7) = -2.049, n.s..
6. Changing Power: Referring to the study above, indicate for each of the following how the
change would affect either sampling error or the treatment effect. Also indicate what would happen to the
size of tobt.
Note: Increasing treatment effect always increases tobt ; Increasing sampling error always decreases tobt.
a.
 treatment effect,  tobt Increasing the length of the training so it would have more impact on participants.
b.
 sampling error,  tobt Decreasing the number of participants.
c.
 sampling error ,  tobt Selecting only participants that started with moderate levels of tolerance.
d.
 sampling error,  tobt Picking managers from several different departments and from very different
working conditions.
e.
 treatment effect,  tobt Making bonuses for managers contingent on improving the tolerance ratings by
subordinates.
7. Picking the correct statistic: Indicate which is the appropriate statistic for the
following situations:
a)
1-sample t-test
Determine whether the average number of community service hours of a particular
fraternity chapter differs from the 5 hour, nation-wide average.
b)
Standard deviation as an estimate, ŝx Estimate the variability in service hours across the entire fraternity based
on the variability of service hours for the local chapter.
c)
Ind. t-test
Compare fraternity and sororities on community service hours. You have 10 members of each.
d)
Mean xbar
Calculate the typical number of twinkies eaten by the 10 fraternity brothers.
e)
z-score (sampling distribution)
Determine the percent of Americans who eat more than the average
number of Twinkies eaten by these fraternity brothers (σ =2).
f)
z-score (frequency distribution) Determine the percent of Americans who eat more than the 92 Twinkies
eaten per day by Big John.
g)
1-sample t-test Determine whether fraternities brothers watch more television than the 3 hour per day, nationwide average. Use your sample of 10 fraternity brothers.
h)
i)
Dependent t-test
Compare 10 football players before and after an all Fried Chicken diet.
Independent t-test Compare 10 football players on the diet for 10 weeks to 10 football players who ate
normally (as normally as football players can eat).