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:
M
d ± (z) (
) where
M
d = M
1- M
2 (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.