What this GraphPad-style t test calculator does
This page gives you a fast, practical way to run the same core analysis you would usually do in GraphPad Prism: a t test comparing two groups. It supports both independent (unpaired) and matched (paired) designs, one- or two-tailed hypotheses, and optional Welch correction for unequal variances.
Paste your data, choose the right settings, and the calculator returns the key outputs most researchers need: t statistic, degrees of freedom, p value, confidence interval for the mean difference, and a simple significance decision.
When to use each t test option
Unpaired t test
Use this when Sample A and Sample B are from different subjects or independent experimental units. Example: control mice vs treated mice.
- Welch correction is safer when group variances may differ.
- Equal SD assumption (classic Student t test) is acceptable when SDs are similar and sample sizes are balanced.
Paired t test
Use this when each value in A is naturally matched to a value in B (before/after measurements, left/right side in same subject, repeated assays on same sample). The test is performed on the within-pair differences.
One-tailed vs two-tailed
- Two-tailed: checks for any difference (A ≠ B). Most common default.
- One-tailed A > B: tests if A is specifically greater than B.
- One-tailed A < B: tests if A is specifically less than B.
How to interpret the output
The most important number is the p value. If p is less than your chosen α (often 0.05), you reject the null hypothesis for your selected tail direction. The confidence interval helps you see the likely range of the true mean difference and whether it includes zero.
- t statistic: signal-to-noise ratio of the observed difference.
- df: degrees of freedom; affects the shape of the t distribution.
- Mean difference: A - B (or paired difference).
- Cohen's d: rough standardized effect size.
Best practices for reliable results
- Check data entry for typos and outliers before testing.
- Use paired tests only when pairs are truly matched.
- Prefer Welch's unpaired test when unsure about equal variance.
- Report exact p values, n per group, and CI in write-ups.
- Remember: statistical significance is not the same as biological importance.
Quick example workflow
- Select Unpaired t test.
- Choose Welch correction.
- Paste control values in Sample A and treatment values in Sample B.
- Leave α = 0.05 and pick a two-tailed hypothesis.
- Click Calculate t test and interpret p, CI, and effect size together.
FAQ
Is this exactly the same as GraphPad Prism?
The calculator reproduces the standard statistical formulas used in GraphPad-style t testing. Prism includes many extra workflow and plotting features; this tool focuses on fast, transparent computation.
Can I paste values separated by line breaks?
Yes. Commas, spaces, semicolons, and new lines are all accepted.
What if one group has very small n?
You can still run the test if minimum requirements are met, but interpretation should be cautious. Very small sample sizes reduce precision and increase uncertainty.