Independent-samples t-test
Compare the mean of a numeric outcome between two separate groups.
When to use it
Use this when each person is in only one of two groups and you want to know whether the group means differ. For example: do exam scores differ between students taught by lecture and students taught by workshop?
If you have three or more groups, use one-way ANOVA. If the same people were measured twice, use the paired-samples t-test.
Assumptions
The outcome is numeric. The groups are separate. The test compares means, so extreme scores can pull the result.
Tensr checks two things and prints them under assumption notes:
- Shapiro–Wilk inside each group. A p-value below .05 means that group’s scores are an uneasy fit to a normal curve. The note then points you to the Mann–Whitney U test.
- Levene’s test (centred at the median) for equal spread. If Levene’s p is .05 or higher, the report uses the equal-variances row. If it is below .05, the report uses the unequal-variances row (Welch).
There is no switch to turn those checks off.
Running it in Tensr
Options
Prop
Type
Reading the output
Lecture versus workshop on the exam. Online is left out so this is a two-group comparison. The workshop mean was built to sit a few points higher, with plenty of overlap.
| Group | n | Mean |
|---|---|---|
| Lecture | 32 | 68.209 |
| Workshop | 32 | 73.741 |
Independent samples t-test of score by method (Lecture vs Workshop) on 64 rows. The primary result is significant (p = .005). This is large enough, in this sample, to treat the comparison this page is about as a real association rather than noise. Metrics: t statistic (Welch) = -2.904; p-value (Welch) = .005; df (Welch) = 56.279; Cohen's d = -0.726; 95% CI (mean diff) = -9.347 to -1.715.
Reporting (APA 7)
Independent samples t-test of score by method (Lecture vs Workshop) on 64 rows. This result is significant (p = .005). The effect is moderate, so report its size with the p-value.
Coming from SPSS
Analyze → Compare Means → Independent-Samples T Test.
SPSS prints both the equal-variances row and the Welch row. Tensr’s Independent Samples Test table prints only the row Levene selected. The metric chips stay labelled Welch.
Related
If normality fails, use the Mann–Whitney U test. Three or more groups belong on one-way ANOVA. The same people measured twice belong on the paired-samples t-test.