# 9.2.1.1 - Minitab Express: Confidence Interval Between 2 Independent Means

Minitab Express can be used to construct a confidence interval for the difference between two independent means. Note that the confidence intervals given in the Minitab Express output assume that either the populations are normally distributed or that both sample sizes are at least 30.

## MinitabExpress – Confidence Interval Between 2 Independent Means

Let's estimate the difference between the mean weight (in pounds) of females and the mean weight of males. Both sample sizes are at least 30 so the sampling distribution can be approximated using the t distribution. Above, we found that the sample standard deviations are similar, so we will assume equal variances.

1. Open the Minitab Express file:
2. On a PC: In the menu bar select STATISTICS > Two Samples > t
3. On a Mac: In the menu bar select Statistics > 2-Sample Inference > t
4. Double click the variable Weight in the box on the left to insert the variable into the Samples box
5. Double click the variable Gender in the box on the left to insert the variable into the Sample IDs box
6. Click OK

This should result in the following output:

2-Sample t: Weight by Gender
 $$\mu_1$$: mean of Weight when Gender = Female $$\mu_2$$: mean of Weight when Gender = Male Difference: $$\mu_1-\mu_2$$

Equal variances are not assumed for this analysis.

Descriptive Statistics: Weight
Gender N Mean StDev SE Mean
Female 126 136.722 23.362 2.081
Male 99 172.717 27.301 2.744
Estimation for Difference
Difference 95% CI for Difference
-35.995 (-42.787, -29.202)
Video Walkthrough

Select your operating system below to see a step-by-step guide for this example.

I am 95% confident that in the population the mean weight of females is between 29.202 pounds and 42.787 pounds less than the mean weight of males.