# Confidence Interval on Difference between Means, Independent Groups, Standard Deviation Known (1 of 3) Following the general formula for a confidence interval, the formula for a confidence interval on the difference between means is:

Md ± (z) ( ) where

Md = M1- M2 (the difference between means) is the statistic and (the standard error of the difference between means) is the standard error.

z depends on the level of confidence desired (1.96 for the 95% interval and 2.58 for the 99% interval).

For a specific example, consider an hypothetical experiment on reaction time (the time it takes to push a button in response to a signal). One group is given a simple reaction time task (a light comes on and they push the button.) A second group is given a choice reaction time task (an "A" or a "B" is presented and the subjects push one button for "A" and a different button for "B"). Assume that it is known that the standard deviation (σ) for the simple reaction time task is 75 and the standard deviation for the choice reaction time task is 100. Five subjects are tested with the simple reaction time task and six subjects with the choice reaction time task. 