How to do it…

The following instructions will create an interval plot showing 95 percent confidence intervals around the mean value.

How it works…

By default, the interval plot displays the data means and the 95 percent confidence intervals around the mean. The results in this graph reveal that supplier B has a wider interval because of greater variation in the data for B:

Double-clicking on Confidence interval as it is circled in the graph will allow us to change the options for this interval as shown in the following screenshot:

We can choose to display Standard error or Confidence interval. Confidence interval can be set as Bonferroni intervals and we can also adjust the confidence level.

The option to pool errors across groups would be used if we knew that sigma, the population standard deviation, is expected to have a similar value between the groups. The confidence intervals plotted here are such that they have an individual error rate of 1- the confidence interval. Bonferroni confidence intervals are used when we want to set the confidence intervals such that the simultaneous error rate across all groups would be a fixed value.

See also