Before you perform a statistical analysis, you can use graphs to explore data and assess relationships between the variables. Also, you can use graphs to summarize data and to help interpret statistical results.
You can access Minitab’s graphs from the Graph and Stat menus. Built-in graphs, which help you interpret results and assess the validity of statistical assumptions, are also available with many statistical commands.
This chapter explores the shipping data worksheet that you opened in the previous chapter. You use graphs to check normality, compare means, explore variability, and examine the relationships between variables.
For more information about Minitab graphs, go to the Help and How-To Overview. In the navigation menu that is on the left, click Graphs. Within the Graphs section, How-To contains information on how to create and interpret specific graphs. Supporting Topics contains general information, such as graph editing and revising graphs.
Before you perform a statistical analysis, first create graphs that display important characteristics of the data. For the shipping data, you want to know the mean delivery time for each shipping center and how the data vary within each shipping center. You also want to determine whether the shipping data follow a normal distribution, so that you can use standard statistical methods for testing the equality of means.
To determine whether the shipping data follow a normal distribution, create a paneled histogram of the time lapse between order date and delivery date.
To select variables in most Minitab dialog boxes, use one of the following methods:
The histograms seem to be approximately bell-shaped and symmetric about the means, which indicates that the delivery times for each center are approximately normally distributed.
For the graph that you created, you want to rearrange the three panels so that comparisons among the means and variations are easier.
The histogram shows that the Central and Eastern centers are similar in both mean delivery time and spread of delivery time. In contrast, the mean delivery time for the Western center is shorter and the distribution is less spread out. Analyzing Data shows how to detect statistically significant differences between means using ANOVA (analysis of variance).
If your data change, Minitab can automatically update graphs. For more information, go to Update graphs when your data change.
To help your supervisor quickly interpret the histogram, you want to change the title and add a footnote.
The paneled histogram now has a more descriptive title and a footnote that provides a brief interpretation of the results.
Graphs can help you identify whether relationships exist between variables, and the strength of any relationships. Knowing the relationship between variables can help you determine which variables are important to analyze and which additional analyses to choose.
Because each shipping center serves a region, you suspect that distance to delivery location does not greatly affect delivery time. To verify this suspicion and to eliminate distance as a potentially important factor, you examine the relationship between delivery time and delivery distance for each center.
To examine the relationship between two variables, you use a scatterplot. You can choose a scatterplot from the Graph menu or you can use the Minitab Assistant. The Assistant guides you through your analyses and helps you interpret the results with confidence. The Assistant can be used for most basic statistical tests, graphs, quality analyses, and DOE (design of experiments).
Use Minitab’s graph layout tool to arrange multiple graphs on one page. You can add annotations to the layout and edit the individual graphs within the layout.
To show your supervisor the preliminary results of the graphical analysis of the shipping data, arrange the summary report and the paneled histogram on one page.
If you edit the data in the worksheet after you create a layout, Minitab cannot automatically update the graphs in the layout. You must recreate the layout with the new graphs.
You want to add a descriptive title to the graph layout.
You can print any Minitab window, including a graph or a layout.
Minitab data are saved in worksheets. You can also save Minitab projects, which contain all of your work, including worksheets, Session window output, graphs, history of your session, and dialog box settings.
The graphical output indicates that the three shipping centers have different delivery times for book orders. In the next chapter, you display descriptive statistics and perform an ANOVA (analysis of variance) to test whether the differences among the shipping centers are statistically significant.
Go to Analyzing Data.