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4.1.5 STEP 5: Find the critical value of t in the t-table
in Appendix E
Objective : To find the critical value of t in the t-table
in Appendix E
Before we get into an explanation of what is meant by “the critical value of t,” let’s
give you practice in finding the critical value of t by using the t-table in Appendix E.
Keep your finger on Appendix E as we explain how you need to “read” that table.
Since the test in this chapter is called the “one-group t-test,” you will use the first
column on the left in Appendix E to find the critical value of t for your research
study (note that this column is headed: “ n” ).
To find the critical value of t, you go down this first column until you find the
sample size in your research study, and then you go to the right and read the critical
value of t for that sample size in the critical t column in the table (note that this is the
column that you would use for both the one-group t-test and the 95% confidence
interval about the mean ).
For example, if you have 27 people in your research study, the critical value of
t is 2.056.
If you have 38 people in your research study, the critical value of t is 2.026.
If you have more than 40 people in your research study, the critical value of t is
always 1.96.
Note that the “critical t column” in Appendix E represents the value of t that you
need to obtain to be 95% confident of your results as “significant” results.
The critical value of t is the value that tells you whether or not you have found a
“significant result” in your statistical test.
The t-table in Appendix E represents a series of “bell-shaped normal curves”
(they are called bell-shaped because they look like the outline of the Liberty Bell
that you can see in Philadelphia outside of Independence Hall).
The “middle” of these normal curves is treated as if it were zero point on the
x-axis (the technical explanation of this fact is beyond the scope of this topic, but
any good statistics book (e.g. Zikmund and Babin 2010) will explain this concept to
you if you are interested in learning more about it).
Thus, values of t that are to the right of this zero point are positive values that use
a plus sign before them, and values of t that are to the left of this zero point are
negative values that use a minus sign before them. Thus, some values of t are
positive, and some are negative.
However, every statistics book that includes a t-table only reprints the positive
side of the t-curves because the negative side is the mirror image of the positive
side; this means that the negative side contains the exact same numbers as the
positive side, but the negative numbers all have a minus sign in front of them.
Therefore, to use the t-table in Appendix E, you need to take the absolute value
of the t-value you found when you use the t-test formula since the t-table in
Appendix E only has the values of t that are the positive values for t.
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