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In Depth Information

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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