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Mathematically, this relationship can be expressed by a simplified formula:
where X 1 and X 2 are the means of the first and second samples, respectively; and
SE is the standard error. The resulting t-value indicates how far the mean of the
difference is from the mean of the t distribution.
Each t distribution is associated with a number that is the sample size minus one,
called the degrees of freedom. After calculating the t-value, you can refer to a table of
significance, look up your degrees of freedom and the level of significance you want
(.05,.01, or .001, for example), and find the critical value of t. If the t-value is larger
than the critical value, then the difference between your two samples is statistically
significant.
Two- or Four-Cell Test
The t-test Waveform Tool contains four cells into which you may enter categories for
comparison. The cells are arranged in a two-column by two-row format. You can enter
categories into two cells or four cells.
Two Cells
X1 – X2 t =
-----------------------------
SE( X1 – X2) With a two-cell t-test, you enter the categories into the columns, not the rows.
Caution!: If you enter categories into the rows and try to save the t-test
specification, an error message appears.
The tool compares the relationship between the two columns. If you enter values such
as:
a b
the tool performs a t-test of the mean of cell a compared with the mean of cell b.
For example, you might have an experiment where subjects read a list of words and
indicate for each word whether it has a positive (“good”) or negative (“bad”)
connotation. You could enter “good” into cell a and “bad” into cell b.
good bad
Net Station Waveform Tools Technical Manual S-MAN-200-WFTR-001 • December 21, 2006
20: T-test
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