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Since you learned how to find the absolute value of t in the previous chapter (see
Sect. 4.1.3.1), you can use that knowledge in this chapter.
5.1.6 STEP 6: Calculate the formula for the two-group t-test
Since we are using two different formulas in this chapter for the two-group t-test
depending on the sample size in the two groups, we will explain how to use those
formulas later in this chapter.
5.1.7 STEP 7: Find the critical value of t in the t-table
in Appendix E
In the previous chapter where we were dealing with the one-group t-test , you found
the critical value of t in the t-table in Appendix E by finding the sample size for the
one group in the first column of the table, and then reading the critical value of
t across from it on the right in the “critical t column” in the table (see Section 4.1.5).
This process was fairly simple once you have had some practice in doing this step.
However, for the two-group t-test, the procedure for finding the critical value of
t is more complicated because you have two different groups in your study, and they
often have different sample sizes in each group.
To use Appendix E correctly in this chapter, you need to learn how to find the
“degrees of freedom” for your study. We will discuss that process now.
5.1.7.1 Finding the degrees of freedom (df) for the Two-group t-test
Objective : To find the degrees of freedom for the two-group
t-test and to use it to find the critical value of t in
the t-table in Appendix E
The mathematical explanation of the concept of the “degrees of freedom” is
beyond the scope of this topic, but you can find out more about this concept by
reading any good statistics book (e.g. Keller, 2009). For our purposes, you can
easily understand how to find the degrees of freedom and to use it to find the critical
value of t in Appendix E. The formula for the degrees of freedom (df) is:
degrees of freedom
¼
df
¼
n 1 þ
n 2
2
(5.1)
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