The independent t-test, also called the two sample t-test or
student's t-test, is an inferential (as opposed to a descriptive) statistical
test that determines whether there is a statistically significant difference
between the means in two unrelated (read: independent) groups.
Independent sample t-test formula
Two formulae are often used:
1: Averaged standard deviations (not used in SPSS)
2: Pooled standard deviations (used in SPSS)
Assumptions
1: Your dependent variable should be measured on a
continuous scale (i.e., it is measured at the interval or ratio level).
2: Your independent variable should consist of two
categorical, independent groups.
3: You should have independence of observations, which means
that there is no relationship between the observations in each group or between
the groups themselves.
4: There should be no significant outliers.
5: Your dependent variable should be approximately normally
distributed for each group of the independent variable.
6: There needs to be homogeneity of variances.
SPSS Guide
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