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Index to Module 4 Notes
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In Module Notes 4.1, we
compared two independent samples of numerical data to determine if, by
inference, the means of the two populations from which the samples were drawn
are equal are not. This situation requires that observations in one sample are
independent from observations in the other sample. To achieve independence, the
observations are randomly drawn and assigned to Sample 1 or Sample 2 in an
unbiased manner (such as by flipping a coin or using a random number
generator).
This module examines the interesting situation in which observations in Sample
1 are related to Sample 2. For our brand of gas car example, a possible
scenario for related samples would be if we ran car number 1 with Brand of Gas
A and recorded mpg. We then flushed the tank and lines, and ran the same
car with Brand of Gas B and recorded mpg.
This experimental design controls for variability within each group. That is,
we may have great variability in two independent samples because we chose BOCs
(big overpriced cars) and CPCs (cheap plastic cars) and mixed them up and put
them both in Group A and Group B. The mpg variability might be so big within
the groups that we might not be able to detect any variability between the
groups. By matching the cars between groups, however, we could control
the variability (Cadillac XYZ is run in Group A, then the same Cadillac is run
in Group B; Neon LMN is run in Group A, then run in Group B; and so forth).
You are probably aware of a lot of tests like this. A student might be given a before
taking a class exam of knowledge, and then the same student is given an after
taking the class exam of knowledge. The professor would hope that the means
would be different!
t-Test for Mean for Paired or Matched Samples
Lets return to the car example. Worksheet 4.2.1 shows the mpg performance
for seven cars assigned to receive Brand A gas. Then the same seven cars
are run with Brand B gas (after purging the tanks and lines). MPG is recorded
for each car. This test involves paired or matched samples since the
first observation in the first group is somehow paired or matched with the
first observation in the second group. In our scenario, it is the exact same
car.
Worksheet 4.2.1
|
Brand A |
Brand B |
|
20 |
20 |
|
20 |
20.5 |
|
19 |
18.5 |
|
16 |
20 |
|
15 |
19 |
|
17 |
19 |
|
14 |
18 |
Now we want to determine if the average mpg performance with Brand A is the
same as that with Brand B. The hypotheses are:
H0:
Mean A = Mean B
Ha: Mean A =/= Mean B
To do this test, we use
another t-Test Data Analysis Add In. Select Tools on the Standard Toolbar
in Excel, Data Analysis in the pulldown menu, then t-Test: Paired Two
Sample for Means. (In Excel 2007 you have to first select Data, then select Data
Analysis, t-Test: Paired Two Sample for
Means). The dialog box is identical to the dialog box for the
t-Tests we did in Module 4.1 Notes. My results are shown in Worksheet 4.2.2.
Worksheet 4.2.2
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t-Test: Paired Two Sample for Means |
||
|
Brand A |
Brand B |
|
|
Mean |
17.3 |
19.3 |
|
Variance |
5.9 |
0.8 |
|
Observations |
7 |
7 |
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Pearson Correlation |
0.6 |
|
|
Hypothesized Mean Difference |
0 |
|
|
df |
6 |
|
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t Stat |
-2.6 |
|
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P(T<=t) one-tail |
0.0198 |
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t Critical one-tail |
1.9 |
|
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P(T<=t) two-tail |
0.0397 |
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t Critical two-tail |
2.4 |
Since the p-value
(two-tail; 0.0397) is less than our threshold value of 0.05, we reject the null
hypothesis and claim that there is a significant difference between the means.
In this case, Brand of Gas B is outperforming Brand of Gas A.
Wait a minute!! When I ran the identical data with a t-Test for Independent
Samples in Module Notes 4.1 we got a p-value of 0.076 and did not reject the
null hypothesis - the means were equal (any difference was due to chance and
not significant). This is a great lesson. We can use statistical test to prove
a point - but it might be the wrong test and it was unethical for us to
use it.
In Module 4.1, when we treated the samples as independent, it doesn't matter
how we enter the data for each sample since the data are independent. But when
we have the situation that the data are related between the two samples (Module
4.2), we are required to enter the pairs in the same row. This allows the
statistical analysis to factor out any variability between rows (also known as
within group variability) and have a controlled comparison between columns
(also known as between group variability).
This is a powerful test procedure that has many applications when there are
paired observations. This wraps up Module 4.2., although I believe I owe you
one more note.
In our tests of means for two independent or paired sample groups, I always
used the two-tailed p-value results because the alternative hypotheses were
non-directional two-tailed. If an alternative hypothesis of interest to you is
Mean A < Mean B or Mean A > Mean B, then you would need to use the
one-tail p-value result.
References:
Anderson, D.,
Sweeney, D., & Williams, T. (2010). Essentials of Business Statistics with
Microsoft Excel. Cincinnati, OH: South-Western, Chapter 10 (Section 10.3).
Ken
Black. Business Statistics for Contemporary Decision Making. Fourth Edition,
Wiley. Chapter 10 & 11
D. Groebner, P. Shannon, P.
Fry & K. Smith. Business Statistics:
A Decision Making Approach, Fifth Edition, Prentice Hall, Chapter 9