Finding the mean to a 95 percent confidence on interval plots_Minitab Cookbook-QQ阅读男生玄幻网

上QQ阅读APP看书，第一时间看更新

Finding the mean to a 95 percent confidence on interval plots

Interval plots are used to display the mean of a group of data and an interval bar around the mean. The intervals can be either standard error bars or confidence intervals.

Here, we will use the Camshaft2.MTW worksheet from the Minitab example data folder. The data shows a recorded length of camshaft from the two suppliers. The interval plot is used to display the mean and confidence interval of the mean for each supplier.

How to do it…

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

Go to the File menu and select Open Worksheet….
Click on the button labeled Look in Minitab Sample Data folder.
Open the Camshaft2.MTW worksheet.
In the Graph menu, go to Interval Plot….
Select the With Groups chart under One Y.
Enter Length into the Graph variables: section.
Enter Supplier into the Categorical variables for grouping (1-4, outermost first): section and then click on OK.

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.