Microsoft Office Tutorials and References
In Depth Information
In other words, you add the sample size for Group 1 to the sample size for Group
2 and then subtract 2 from this total to get the number of degrees of freedom to use
in Appendix E.
Take a look at Appendix E.
Instead of using the first column as we did in the one-group t-test that is based on
the sample size, n, of one group, we need to use the second-column of this table (df)
to find the critical value of t for the two-group t-test.
For example, if you had 13 people in Group 1 and 17 people in Group 2, the
degrees of freedom would be: 13 þ 17 2 28, and the critical value of t would be
2.048 since you look down the second column which contains the degrees of
freedom until you come to the number 28, and then read 2.048 in the “critical
t column” in the table to find the critical value of t when df
28.
As a second example, if you had 52 people in Group 1 and 57 people in Group 2,
the degrees of freedom would be: 52
107 When you go down the
second column in Appendix E for the degrees of freedom, you find that once you go
beyond the degrees of freedom equal to 39, the critical value of t is always 1.96 , and
that is the value you would use for the critical t with this example.
þ
57
2
5.1.8 STEP 8: State the result of your statistical test
The result follows the exact same result format that you found for the one-group
ttest in the previous chapter (see Section 4.1.6):
Either: Since the absolute value of t that you found in the t-test formula is less than
the critical value of t in Appendix E, you accept the null hypothesis.
Or: Since the absolute value of t that you found in the t-test formula is greater than
the critical value of t in Appendix E, you reject the null hypothesis and accept the
research hypothesis.
5.1.9 STEP 9: State the conclusion of your statistical test
in plain English!
Writing the conclusion for the two-group t-test is more difficult than writing the
conclusion for the one-group t-test because you have to decide what the difference
was between the two groups.
When you accept the null hypothesis, the conclusion is simple to write: “There is
no difference between the two groups in the variable that was measured.”
But when you reject the null hypothesis and accept the research hypothesis, you
need to be careful about writing the conclusion so that it is both accurate and
concise.
Let’s give you some practice in writing the conclusion of a two-group t-test.
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