
Chapter 11
370
Note: The z formula for the difference between the mean values of two populations can also be
manipulated to produce a formula for constructing the con dence intervals for the difference in
two population means. So, the con dence interval to estimate the difference in two population
means can be presented as follows:
Con dence interval to estimate the difference in two population means
() ()xx z
nn
xxz
12
1
2
1
2
2
2
12 12
1
2
1
2
2
2
−− +≤−≤ −+ +
σσ
μμ
It has already been discussed that when population standard deviations are unknown, sample
standard deviations are good approximations of population standard deviations if the sample
sizes are ...