A concrete example should make the procedure for computing the confidence interval clearer. Assume that an experimenter were interested in computing the 99% confidence interval on the difference between the memory spans of seven- and nine-year old children. Four children at each age level are tested and their memory spans are shown below:

Age 7 |
Age 9 |

3 3 4 5 |
5 6 6 7 |

The first step is to compute the means of each group:

M

Therefore, M

To obtain a value of t, one must first compute the degrees of freedom (df). The degrees of freedom is equal to the degrees of freedom for MSE (MSE is used to estimate σ²). Since MSE is made up of two estimates of σ² (one for each sample), the df for MSE is the sum of the df for these two estimates. Therefore, the df for MSE is (n -1) + (n - 1) = 3 + 3 = 6.