Confidence Intervals for μ (when σ is unknown)

 

Suppose that you want to determine the average weight of Bears collected by Gary Alt as reported by the Readers Digest in 1979.

This data set is located as a worksheet on the Minitab CD in the Students12 folder called Bears2.

When sigma is unknown but n is larger than 30, you can conduct a CI that uses z.

(It is also acceptable to use the t distribution.)

  1. Find the sample standard deviation by using the descriptive statistics command. In this case, s = 117.8.
  2. To do this, enter your data into C1. Then go to Stat, Basic Statistics, 1 sample z.
  3. Your screen should look like this.

 

  1. Enter the variable that you want and enter 117.8 in the standard deviation box.  
  2. Now click on Options.


 

  1. Make sure that you entered the correct confidence level.
  2. Next pick the type of confidence interval or bound.
    1. If you want a confidence interval select, "not equal" as your alternative.
    2. If you want a confidence bound, select "less than" for a upper bound or "greater than" for an lower bound.
  3. Your output may look like the following.

One-Sample Z: Weight

 

The assumed standard deviation = 117.8

 

 

Variable   N     Mean    StDev  SE Mean        95% CI

Weight    50  202.840  117.801   16.659  (170.188, 235.492)