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Alternately, if you wrote that: ‘‘Female alumni were significantly less satisfied
than male alumni,’’ the end of this conclusion should be: (4.37 vs. 7.26) since you
mentioned females first, and males second.
Putting the two mean scores at the end of your conclusion saves the reader from
having to turn back to the table in your research report to find these mean scores to
see how far apart the mean scores were.
Now, let’s discuss FORMULA #1 that deals with the situation in which both
groups have a sample size greater than 30.
Objective: To use FORMULA #1 for the two-group t-test when both groups
have a sample size greater than 30
5.2 FORMULA #1: Both Groups have a Sample Size
Greater than 30
The first formula we will discuss will be used when you have two groups with a
sample size greater than 30 in each group and one measurement on each member
in each group. This formula for the two-group t-test is:
t ¼ X 1 X 2
S X 1 X 2
ð 5 : 2 Þ
S 1
n 1
þ S 2
n 2
where S X 1 X 2 ¼
ð 5 : 3 Þ
and where degrees of freedom = df ¼ n 1 þ n 2 2
ð 5 : 1 Þ
This formula looks daunting when you first see it, but let’s explain some of the
parts of this formula:
We have explained the concept of ‘‘degrees of freedom’’ earlier in this chapter,
and so you should be able to find the degrees of freedom needed for this formula in
order to find the critical value of t in Appendix E.
In the previous chapter, the formula for the one-group t-test was the following:
t ¼ X l
ð 4 : 1 Þ
where s : e : ¼ S X ¼ S
ð 4 : 2 Þ
For the one-group t-test, you found the mean score and subtracted the
population mean from it, and then divided the result by the standard error of the mean
(s.e.) to get the result of the t-test. You then compared the t-test result to the critical
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