Paired-Samples T-Test
measure the same group of
people twice.
Assumptions of the paired-samples t-test
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interval or continuous
independent.
normally distributed.
equal variances.
T-Value
§ The t-test produces a value, simply known as ‘t’,
which can only be used to determine statistical
significance.
§ It does not say anything about the size of the
difference between the two mean scores (i.e., the
effect size).
§ Researchers have to run a separate effect size
calculation to obtain Cohen’s d.
The Effect Size for the pairedsamples t-test
§ Cohen’s d is an effect size index for t-tests.
§ Cohen’s d-value = (Mean 1 – Mean 2) ÷ pooled
standard deviation.
§ To determine effect sizes, researchers either have
to compute them by hand or use a web-based
calculator, such as Becker’s at
http://www.uccs.edu/~lbecker/
Steps to Perform a pairedsamples t-test
§ Step 1: Examine and evaluate the descriptive
statistics of the data from two groups and the
reliability of the research instrument(s) being
used.
§ Step 2: Check whether the statistical assumptions
for the particular t-test are met. Levene’s test for
equality of variances can be used to determine
whether the two means have equal variances
(SPSS can perform this statistical test; see below).
Paired-Samples T-test in SPSS
§ Click the Analyze menu. Go to Compare Means and then PairedSamples T Test
§ On clicking Paired-Samples T Test, a dialog box opens
§ First, select the variables ‘implicature score’ and ‘routines score’.
Make sure they are paired up as Variable 1 and Variable 2.
§ Click Continue and OK.
Cohen’s d