The solution is to use the harmonic mean of the two sample sizes for n. The harmonic mean (n

Therefore the formula for the estimated standard error of the difference between means is:

For the example of the confidence interval on the difference between memory spans of seven-year olds and nine-year olds, assume that one more seven year old was tested and the resulting memory span score was 3.

For these data, M

SSE

(4 - 3.6)² + (5 - 3.6)² = 3.2.

SSE