95% CI and Width for Mean # of hrs watching
TV for Three Different Sample Sizes
Sample Size
Confidence
Interval
Interval Width
Stand Dev
Stand Error
N=195
2.65-3.27
0.62
2.26
0.16
N=504
2.76-3.16
0.40
2.26
0.10
N=1,987
2.86-3.06
0.20
2.26
0.05
Part IV
Significantly Different
Using Inferential
Statistics
Chapter 11   
t(ea) for Two (Again)
Tests Between the Means of Related Groups
What you will learn in Chapter 11
When to use a t test for independent
means
 How to compute the observed t value
 Interpreting the t value and what it means

t Tests for Dependent Samples

Determining the correct statistic
Computing the Test Statistic

Numerator reflects the sum of the
differences between two groups
Degrees of Freedom
Degrees of freedom approximate the
sample size
 Degrees of freedom can vary based on the
test statistic selected
 For this procedure…

n
– 1 (where n is the number of observations)
Doing the t test
Chapter 7
Prozac Group
Placebo Group
53
64
42
68
58
84
39
65
40
50
50
60
61
81
44
76
7
Doing the t test
Steps in the process

Step 1:
 Calculate

Step 2:
 Calculate
Chapter 7
x1
x2
8
Doing the t test
Steps in the process

Step 3:
 Count
the number of scores in each group.
N1 = # scores in Group 1.
 N2 = # scores in Group 2.

9
Chapter 7
Doing the t test
Steps in the process

Step 4:
 Calculate
sample variance for both groups
Step 5: Do the math!! Be sure to solve within
brackets first!!
10
Chapter 7
Significance testing
Steps in the process

Step 1:
 Calculate


N1 + N2 – 2
Step 2:
 One
11
the degrees of freedom
or two-tailed test of significance?
Did you have a directional hypothesis or not?
Chapter 7
Significance testing
Steps in the process

Step 3:
 Using
the table, find the critical values of t
using N1 + N2 – 2 degrees of freedom.
 Page 353-355, the t distribution.

Step 4:
 Compare
the obtained t with the critical value
of t.
12
Chapter 7
You Try!!
13
Participant
ID
Placebo
Participant
ID
Treatment
13
59
11
28
14
37
12
34
15
42
17
26
20
30
18
36
23
37
19
23
24
51
22
24
29
55
26
36
30
46
27
26
Chapter 7
So How Do I Interpret…
t
(24)
= 2.45, p > .05
t represents the test statistic used
 24
is the number of degrees of freedom
 2.45 is the obtained value (from the formula)
p > .05 indicates the probability
Using the Computer

SPSS and Pared Samples t Test
SPSS Output

What does it all mean?