Contents
How do you find the equal or unequal variance for a t test?
There are two ways to do so:
- Use the Variance Rule of Thumb. As a rule of thumb, if the ratio of the larger variance to the smaller variance is less than 4 then we can assume the variances are approximately equal and use the Student’s t-test.
- Perform an F-test.
What is unequal variance t test?
The unequal variance t test reports a confidence interval for the difference between two means that is usable even if the standard deviations differ. Both t tests report both a P value and confidence interval. The calculations differ in two ways: Calculation of the standard error of the difference between means.
What is the degree of freedom for a t-test for the difference between two means?
Estimating the Standard Error of the Difference Between Means. If σ12= σ22, the pooled variance, Sp, may be used to estimate the common variance, σ2. The degrees of freedom for the t-test in this case are n1+n2-2.
When to use Welch’s t test for unequal variance?
Observation: This theorem can be used to test the difference between sample means even when the population variances are unknown and unequal. The resulting test, called, Welch’s t-test, will have a lower number of degrees of freedom than (nx – 1) + (ny – 1), which was sufficient for the case where the variances were equal.
How many degrees of freedom are there with equal variance?
If s 1 happens to be equal to s 2 and n 1 = n 2 = n, this reduces to 2 ( n − 1) = 2 n − 2, i.e. the same number of degrees of freedom you would have with an equal variance t -test. For your example n = 11, so you would get 20 degrees of freedom, similar to your 19 degrees. So I would guess that your two standard deviations are very similar.
Which is better the equal or the unequal t test?
If the variances are equal then the equal and unequal variances versions of the t-test will yield similar results (even when the sample sizes are unequal), although the equal variances version will have slightly better statistical power. Observation: The calculation of the effect size and the effect size confidence interval is the same as for
How to calculate degrees of freedom for Welch’s test?
Statistical packages estimate the degrees of freedom for the Welch’s t-test, or more simplistically (and less accurately) we can estimate the degrees of freedom by subtracting one from the smaller of the two sample sizes (in this case 99).