two sample t-test unequal variance degrees of freedom
Student's t-test
A typical example of the repeated measures t - test would be where subjects are tested prior to a treatment , say for high blood pressure , and the same subjects are tested again after treatment with a blood - pressure lowering medication . By comparing the same patient 's numbers before and after treatment , we are effectively using each patient as their own control . That way the correct rejection of the null hypothesis ( here : of no difference made by the treatment ) can become much more likely , with statistical power increasing simply because the random between - patient variation has now been eliminated . Note however that an increase of statistical power comes at a price : more tests are required , each subject having to be tested twice . Because half of the sample now depends on the other half , the paired version of Student 's t - test has only n / 2 − 1 degrees of freedom ( with n being the total number of observations ) . Pairs become individual test units , and the sample has to be doubled to achieve the same number of degrees of freedom . Normally , there are n − 1 degrees of freedom ( with n being the total number of observations ) .
A typical example of the repeated measures t - test would be where subjects are tested prior to a treatment , say for high blood pressure , and the same subjects are tested again after treatment with a blood - pressure lowering medication . By comparing the same patient 's numbers before and after treatment , we are effectively using each patient as their own control . That way the correct rejection of the null hypothesis ( here : of no difference made by the treatment ) can become much more likely , with statistical power increasing simply because the random between - patient variation has now been eliminated . Note however that an increase of statistical power comes at a price : more tests are required , each subject having to be tested twice . Because half of the sample now depends on the other half , the paired version of Student 's t - test has only n / 2 − 1 degrees of freedom ( with n being the total number of observations ) . Pairs become individual test units , and the sample has to be doubled to achieve the same number of degrees of freedom . Normally , there are n − 1 degrees of freedom ( with n being the total number of observations ) .