TUTORIAL MINITAB 15

The story

You work for a company that produces pressure treated lumber products. Chromated copper arsenate, or CCA, is commonly used by your industry to make wood less susceptible to rot and insect damage. However, CCA has come under attack by groups worried about potential health risks associated with lumber treated in this manner. You are investigating the viability of a new, entirely organic compound that may be able to replace CCA, maintaining the quality standards of your products and satisfying public interest groups. You have conducted a study and saved the results in the worksheet PRESSURE.MTW:

In your study, you attempted to simulate how well CCA and the new organic compound protect wood over a 60 year period. Uniform planks of pine were treated with both products at three standard saturation levels and then placed in an aging chamber. The planks were then tested for their load bearing properties to see how well the different treatment regiments preserved the wood.

The variable Solution recorded whether the product used was CCA (Solution = 1) or organic (Solution = 2).

Retention states the amount of solution applied to each plank: 0.25 pounds of preservative per cubic foot of wood, the standard for above ground applications, 0.8 lbs/cubic foot, the standard for foundation piles, and 2.5 lbs/cubic foot, the standard for salt water applications.

The values listed in Hours are the number of hours each plank spent in the aging chamber. One hour is considered equivalent to one year of exposure to the elements.

Load lists the maximum load weight in pounds for each plank following the preservative treatment and aging process.

You have been asked to present these findings to your group manager and have set the following objectives for yourself:

    Create two graphs that best represent the data.

    Edit the graphs for clarity.

    Combine the graphs in a layout.

    Prepare a short report discussing what the graphs say about the data, and include the graphs in this document.

What you will learn

In this session, you will learn how to:

    Choose an appropriate graph.

    Apply common graph options when creating a graph.

    Explore graphed data without altering the graph.

    Select and edit the attributes of different graph items.

    Alter the variable used to create a graph.

    Add items to a graph.

    Use a grouping variable to alter the display of a graph.

    Create a layout of several graphs.

    Edit a Minitab graph in other software applications.

    Use command language to create graphs.

Time required

About 50 minutes.

Proceed with:

Step 1: Start a New Project.

Step 2: Open a Worksheet

Step 3: Choosing the Right Graph

Step 4: Using a Graph Gallery

Step 5: Entering Data into a Graph Dialog Box

Step 6: Common Graph options

Step 7: Initial Graph Exploration

Step 8: Brushing

Step 9: Adding a Paneling Variable

Step 10: Selecting Graph Items for Editing

Step 11: Basic Graph Editing

Step 12: Adding a Graph Element

Step 13: Changing Worksheet Data and Replacing Variables

Step 14: Creating a Grouped Boxplot

Step 15: Advanced Graph Editing

Step 16: Create a Layout

Step 17: Graphs in Other Applications

Step 18: Graphs and Command Language

Step 19: Save and Exit

Step 20: What You've Learned

Step 1: Start a New Project

1    Choose File > New.

2    Choose Project, then click OK.

During the course of this tutorial you will create graphs, edit your worksheet, and more. If you need to stop before completing the session, you'll want to save your work as a Project. When you save your project, you save all your work at once: all the data, all the output in the Session window, and all the open Graph windows. When you reopen the project, all that information will be waiting for you, right where you left it. Follow these steps for saving your project:

1    Choose File > Save Project As.

2    In Save in, navigate to the location where you'd like to keep your project.

3    In File name, enter a name for your project, and click Save.

After this, simply choose File > Save Project to save all of your work.

If you want to use output or data in another application or another Minitab project, you can save your Session window output, data, and graphs as separate files. These separate files are copies of what is currently in your projectthe contents of your project are not changed in any way.

Step 2: Open a Worksheet

When you start Minitab, you begin with a new, empty project. You can add data to your project in many ways, but the most common way is to open a worksheet. Note that you are only copying the data from the worksheet to the project; any changes that you make to the data added to your project will not affect the original file.

In this session, you will use the file PRESSURE.MTW. This file is one of the dozens of worksheets that are shipped with Minitab. Most of these worksheets are in the Data subdirectory or folder.

1    Choose File > Open Worksheet.

2    Click Look in Minitab Sample Data folder, then select PRESSURE.MTW.

3    Click Open, then OK (if necessary).

Step 3: Choosing the Right Graph

Graphs allow you to display patterns, relationships, and distributions in your data that are difficult to evaluate simply by looking at a worksheet. Which graph you choose depends on the type of data you have to analyze, and what you hope to show by graphing it. For your first graph, you want to show the relationship between the relative strength of pressure treated wood and the length of time is has been exposed to nature. Use the table below to find the best graph for this goal.

You are interested in the relationship between a pair of variables, Load and Time. Because one of your variables is a measurement of time, you might be tempted to use a time series plot, but remember that your data were not collected over time, but at different times. You might measure the stock market over time or a single patient's heart rate over time. In your study you have collected information about many different blocks of wood at different times, not a single block over time. You choose to draw a scatterplot.

1    Choose Graph > Scatterplot.

Step 4: Using a Graph Gallery

After you choose the type of graph you want to create, you must narrow your selection with a graph gallery. These galleries offer common graph variations allowing you to create a graph that is tailored to your needs with minimal input. Gallery options vary to suit the graph being created, but they commonly offer the following versions:

   Simple - for use without categorical grouping variables

   With Groups - for use with categorical grouping variables

   Multiple Y's - when including more than one variable on a categorical graph

   A version that adds a data display or fitted line to one of the above choices

At this point, you're unsure of the need for regression or connect lines, but you do want to utilize the grouping variables Solution and Retention. You decide on the With Groups option.

1    Choose With Groups, and click OK.

Note Some graph gallery choices are based on the number and type of variables being used (Simple, With Groups, Multiple Y's), and others simply add an item (With Regression, With Connect Line). If you are unsure of the gallery option that's right for your needs, focus first on the variables that will be used to create the graph.

Step 5: Entering Data into a Graph Dialog Box

The Scatterplot - Data Source dialog box appears, prompting you for the variables to use in your graph.

A thick, black line around the first row of the Y column (the active location) indicates that a variable selected from the list will be placed here. To insert a variable into the active location, click a variable in the variable list box and click Select, or simply double-click the variable. For this graph, you want to plot each plank's maximum load weight against the time it spent in the aging chamber, and group the data by the solution used and the retention level.

1    Under Y variables, enter Load. Under X variables, enter Hours.

2    In Categorical variables for grouping (0-3), enter Solution.

Notice the Scale, Labels, Data View, Multiple Graphs, and Data Options buttons. These buttons access common graphing options, tailored to the current graph, that you may want to use when creating your graph. You'll explore these options in the next step.

Step 6: Common Graph Options

Depending on the graph, you may not have the common graph options Scale, Labels, Data View, Multiple Graphs, and Data Options, and you may have others exclusive to that graph. With these options, you can make many adjustments to the default look of the graph. The options available with these buttons vary to suit the graph being created, but in general these buttons control:

  Scale: Axes and ticks, gridlines, reference lines, and percentile lines

   Labels: Data labels, titles, and footnotes

   Data View: Data display items including fitted lines

   Multiple Graphs: Multiple graph placement and paneling variables

   Data Options: Subsetting data, handling missing values, and frequency columns

Because your graph will include so many data points, you'd like to use minor ticks.

1    Click Scale.

2    Under Y Scale, check Minor Ticks for Low.

3    Under X Scale, check Minor Ticks for Low. Click OK in each dialog box.

Graph window output

Immediately, you can see patterns in your data. In the next step, you'll begin to explore these patterns.

Step 7: Initial Graph Exploration

In order to write an accurate description of the graphs in your report, you'll want to examine them as thoroughly as possible. A quick visual inspection of the graph tells you that, in general, wood treated with CCA retained its strength longer than wood treated with the organic solution. You can easily investigate further. Data tips and crosshairs allow you to see exact values of the given variables and thus make more accurate observations in your report.

Data Tips

1    Place the cursor over any point on the graph.

A data tips window appears, giving the row and coordinates of the point. If the point was an extreme outlier, you might want to examine it further. This technique works for reference lines, percentile line, and other graph items- just place the cursor over the item in question and a data tip appears.

Crosshairs

1    If the graph editing toolbar is not already active, choose Tools > Toolbars > Graph Editing.

2    Click , the crosshairs tool.

3    Move the cursor across the data region.

Exact coordinates are given as you move the crosshairs. You can now refine your earlier generalizations about the differences between observations treated with different solutions.

In the next step, you will examine brushing as a method for learning even more about your graph points.

Step 8: Brushing

Set ID Variables

You suspect that the bands of observations in your graph correspond to the different retention levels used in the experiment.

1    Choose Editor > Set ID variables

2    In Variables, enter Retention. Click OK.

Now values for the variable Retention will be displayed for every point you brush.

Brushing a Single Point

1   Place the cursor (now a pointed finger) over the topmost point in the graph and click the mouse.

Now you know that the retention level for this observation is 2.5. You could brush many individual points across the graph to get an idea of the most common retention level in each band, or you could create a brushing region.

Create a Brushing Region

1    Place the pointed finger in the upper left corner of the graph.

2    Click and hold the mouse and drag the cursor down and to the right side of the graph, enclosing the top two points in the brushing region

3    Slowly drag the brushing region from left to right along each band of observations and observe the values under Retention in the brushing palette.

4    To leave brushing mode, click on the graph editing toolbar.

It is clear that these bands represent the different retention levels applied with each solution.

Brushing can help you to better understand your data, and may persuade you to restructure your graph. In this case, brushing helped confirm your suspicion that the stratification in observations was caused by the different retention levels. In the next step, you'll examine how to incorporate this variable into your graph.

Step 9: Adding a Paneling Variable

You have shown the relationship between a plank's maximum load weight and the time it spent in the aging chamber, and grouped the data points by the solution used. Brushing has shown that the different retention levels are responsible for the rather dramatic stratification in your graph's data points. Because Retention is categorical, you might want to use it as a grouping variable. While this wouldn't be incorrect, it would be appropriate in this case to use Retention as a paneling variable. So, what's the difference between a grouping variable and a paneling variable? It's a matter of how the graph is displayed, as you'll see in the example below.

Grouping Variable

In the graph to the right, the categorical variable Color has been used as a grouping variable. Color has two values, black and red, and two symbol types are represented on the graph. All of the points share the same data region, regardless of their group value. This type of graph is useful for showing how groups interact.

Paneling Variable

This graph is identical to the one above it, except that Color has been used as a paneling variable instead of a grouping variable. The points are displayed in a separate graph panel for each value of the categorical variable. This type of graph is useful for showing how groups compare and contrast.

1    Choose Editor > Panel.

2    In By variables with groups in separate panels, enter Retention.

3    Click OK.

Graph window output

Your graph is still grouped by Solution, but now it is paneled by Retention. It is now much simpler to describe the differences between solutions at different retention levels.

Note You can use a paneling variable during graph creation by choosing the Multiple Graphs button, and then choosing the By Variables tab.

Step 10: Selecting Graph Items for Editing

You can edit essentially every element on a Minitab graph. You can make cosmetic changes, like the font of a title or the color of a line, or more structural changes, like increasing the range of a scale or fitting a Lowess line. But first, you must select the item you want to edit. The three methods for selecting an item are:

   Click the item.

   Choose Editor > Select Item, then choose the item from the list.

   Select the item from the list in the graph editing toolbar.

Depending on the item, one method might be simpler than another. A special case is evidenced in the scatterplot you created in step 5. If you click any point on the graph once, all points are selected. Click the same point a second time and only the points in that group are selected. Click the point a third time and the individual point is selected.

After you select the intended item, you can begin editing it in one of four ways:

   Double-click the item.

   Choose Editor > Edit.

   Right-click and choose Edit.

   Click , the Edit button, on the Graph Editing toolbar.

Step 11: Basic Graph Editing

Graph items, such as data regions, tick labels, and many others, adjust automatically to produce the clearest possible graph by default. Even so, you can alter the look of these items to suit you needs.

Panel arrangement

Change the way the graph panels are arranged and spaced.

1    Double-click a panel label.

2    Click the Arrangement tab.

3    Under Rows and Columns,

choose Custom.

4    In Rows enter 1 and in Columns enter 3.

5    In Margins between panels, enter 0.02.

6    Click the Options tab.

7    Under Alternate Ticks On Panels When Appropriate, choose Don't Alternate Panels, and click OK.

Graph title

Increase the size and change the title text.

1    Double-click the graph title.

2    In Size, enter 16.

3    In Text, delete the default graph title. Enter Maximum Load Over Time.

4    Click OK.

Scale, tick labels, and axis label

Each hour in the aging chamber represented six months of real time exposure to the elements. Adjust the scale, tick labels, and axis label to reflect the simulated time scale.

1    Double-click the x-scale.

2    Under Major Tick Positions, choose Position of ticks and type 0 15 30 45 60.

3    Click the Labels tab.

4    Under Major Tick Labels, choose Specified and type 0 15 30 45 60 and click OK.

5    Double click the x-axis label.

6    In Text type Simulated Years of Exposure and click OK.

When editing a graph, you are not limited to altering the appearance of symbols, labels, and lines. In the next step you'll practice adding items.

Step 12: Adding a Graph Element

In step 6, you explored the use of common graph options when creating a graph. Among the many options, you can add labels, lines, and markers that make your graph clearer and more expressive. You can also add these items to a graph after it is created. There are three ways to add a graph element:

   Choose Editor > Add.    Right-click the graph, then choose Add.    From the Add List on the Graph Editing toolbar , choose the item you want to add. If this toolbar is not currently displayed, choose Tools > Toolbars > Graph Editing.

The items you can add differ from graph to graph, just as the items in the various common graph options are tailored to each graph. Because you've separated your data points into three panels, you decide to add y-axis gridlines to make it easier to gauge the y-value of points far from the y-axis tick labels.

1    From the list of elements in the Graph Editing toolbar, choose Gridlines.

2    Under Show Gridlines For, choose Y major ticks.

3    Click OK.

Graph window output

You could have added these gridlines at the initial graph gallery, through a common graph option (Scale > Gridlines) or as they were here with Editor > Add.

Step 13: Changing Worksheet Data and Replacing Variables

In the last few steps you've seen how easy it is to alter and augment the look of a graph elements. But from time to time, you might also need to alter or augment the data used to create the graphs. You might need to correct a misrecorded observation, or you may simply collect more data and add it to your worksheet. In the case of open database connectivity (ODBC), changing worksheet data is a given. No matter the reason, when you change data, any graph created with the altered variables will be incorrect. Minitab alerts you to these discrepancies and allows you to update a graph that no longer represents its worksheet.

In the worksheet PRESSURE.MTW, the categorical variable Solution was coded to simplify data entry. A value of 1 stands for CCA, 2 for the organic solution. The legend is created directly from these coded values in the worksheet, so the legend would mean little to anyone not familiar with way the data were coded. The only way to change these labels is to change the worksheet data.

1    Choose Data > Code > Numeric to Text.

2    In Code data from columns, enter Solution.

3    In Into columns, enter Solution.

4    In the first row of Original values, type1.

5    In the first row of New, type CCA.

6    In the second row of Original values, type 2.

7    In the second row of New, type Organic, and click OK.

The column Solution now contains more descriptive values. Notice that the icon in the top left

corner of the graph window has changed from (graph accurately reflects worksheet) to (graph does not reflect current worksheet).

1    Choose Editor > Update > Update Graph Now.

The legend has changed to match the more descriptive values in Solution, and the update icon has returned to green.

Replacing graph variables

You have just seen how to replace individual data without re-creating the graph. You can also replace entire graphing variables. At times, you may create and edit a graph only to find that you want to use a different grouping or paneling variable, or even the graphing variables themselves. Choosing Editor > Create Similar Graph will allow you to replace any of the variables used to create a graph without re-entering information in a graph data source dialog box, and without losing any of your editing changes.

You've ended up with an attractive, very descriptive graph. You might now consider pursuing more statistical analyses along these lines. You have shown that CCA consistently outperformed the organic compound, and that observations followed a rather clear pattern. Statistical analysis can help you decide if these differences are statistically significant, and if the trends they demonstrate will allow accurate predictions of performance. But first, let's apply what we've covered in creating your second graph.

Step 14: Creating a Grouped Boxplot

For your second graph, you want to display the distribution and median of the maximum load weights grouped by solution. You must compare these groups based on the three possible retention levels. Breaking this objective into steps will help you to create the proper graph.

Choose the right graph

First, you know that the graph must assess distributions and display median values. You consult the chart in Step 3 and choose boxplot. You also know that the single variable, Load, will be grouped and so you choose accordingly in the graph gallery.

1    Choose Graph > Boxplot.

2    Choose One Y - With Groups, then click OK.

Enter the variables

Enter the graph variable and grouping variable in the data source dialog box.

3    In Graph Variables, enter Load.

4    In Categorical variables for grouping, enter Solution.

Use graph option buttons

You need to divide the grouped observations into two panels based on the values in Retention.

5    Click Multiple Graphs.

6    Click the By Variables tab.

7    In By variables with groups in separate panels, enter Retention.

8    Click OK in each dialog box.

Graph window output

You decide to make some alterations to clarify the graph for your report.

Step 15: Advanced Graph Editing

Now that you've created your second graph, take some time to examine it. You notice that the title and panel arrangement don't seem consistent with the scatterplot you created. As you explore the graph further you notice (because of the data tips) that exact median values are not immediately apparent in the graph. These are the kinds of things that may influence your editing.

Panels

Place tick labels on the same side and create a margin between panels.

1    Double-click a panel label.

2    Click the Arrangement tab.

3    Under Rows and Columns, choose Custom.

4    In Rows, type 1 and in Columns type 3.

5    In Margins between panels enter 0.02.

6    Click the Options tab.

7    Under Alternate Ticks on Panels When Appropriate, choose Don't alternate panels and click OK.

Axis label and title

Change the title to match the scatterplot you created. The x-axis label can be deleted because the x-axis tick labels are so descriptive.

4    Double-click the title.

5    In Text, type Distribution of Maximum Load Weights.

6    In Size enter 14 and click OK.

7    Select the x-axis label and click .

Add and edit data labels

Add median labels and change their display characteristics.

1    Choose Editor > Add > Data Labels.

2    Click the Medians tab.

3    Under Label type choose Use y-value labels and click OK.

4    Double-click a median label.

5    Click the Font tab.

6    From Color, choose .

7    Under Style, check Bold.

8    In Size, enter 18.

9    Click the Alignment tab.

10    Under Offset, choose Custom.

11    In Horizontal, enter 0.02 and click OK.

Your boxplot is complete, but you may wish to continue your inquiry into the distribution of your data with various stat procedures. The graph may have made you question just how different the group medians really are, or if variance in the spread of group observations is cause for concern.

Now that you have the graphs just as you want them, it's time to move on to the next objective: creating a graph layout.

Step 17: Graphs in Other Applications

For one of your final objectives in the assignment, you need to add your graph layout to a document that discusses the findings of your graphs. While the ReportPad allows you to create reports in Minitab, you decide to use Microsoft Word to generate your report.

Whether you choose to copy a graph into a word-processing document or a slide-show presentation, the process is essentially the same. Depending on the method you use, a Minitab graph copied into another application retains all of its editable properties.

1    With the layout active, choose Edit > Copy Graph.

2    In the Word document, place the cursor where you want to insert the graph.

3    Choose Edit > Paste Special.

If you use a different word processor, you may not have the Paste Special option. In this case, you can still use Paste, but you do not have access to the options discussed in the next step.

4    In As, choose Mtb Graph Object. This ensures that the graph can be edited in the Word document. With other options (picture or bitmap, for example), the resulting image cannot be edited with Minitab graph editing tools.

5    Click OK.

6    Double-click the graph in the Word document. Minitab's graph annotation toolbar appears, and you can double-click any graph item to edit it as you would in Minitab.

Step 18: Graphs and Command Language

This tutorial has focused on using menu items and dialog boxes to construct and edit graphs. You can use command language instead either out of personal preference or to create time-saving macros. A number of menu items can help you become more familiar with session commands, and can simplify your use of command language, especially when creating complex graphs. For basic instruction in using command language, see Help > Session Command Help.

Enable commands

You can set the Session window to display session commands as you create graphs.

1    With the Session window active, choose Editor > Enable Commands.

Now, you can enter session commands in the session window. When you use menus to create a graph, the corresponding command language appears in the Session window. Notice, however, that any editing you do is not reflected in the Session window. After you edit a graph, choose Editor > Duplicate Graph. The Session window displays all of the session commands used to create the edited graph.

Copy command language

Even if you are quite proficient with Minitab session commands, you may find it simpler to create a complex graph using menu items. You can then use the resulting graph to generate session commands.

1    Recall the boxplot you created in this tutorial.

2    With the Graph window active, choose Editor > Copy Command Language.

The command language used to create the graph has now been copied to the clipboard. You cannot copy the command language used to create a layout in this manner.

Command line editor

You can paste the session commands from the clipboard into ReportPad or another application to construct your macro, or directly into the command line editor.

1    Choose Edit > Command Line Editor.

2    Right-click and choose Paste.

3    Edit the commands if you choose, and click Submit.

Minitab uses the session commands to create the graph.

Step 19: Save and Exit

You've completed all of the work you need to do in Minitab for your presentation. Recall from Step 1 that you need to save this project so you can return to the worksheet you used and any graphs you created just as you left them.

1    Choose File > Save Project.

2    Exit Minitab by choosing File > Exit.

Remember that if you prepared your presentation with ReportPad instead of a word processing application, all of your work would be contained and saved in this one project. The final step will recap everything that has been covered in this session.

Step 20: What You've Learned

You have come to the end of the graphics tutorial. Take a moment to review what has been covered and revisit any areas with which you are not yet comfortable. In this tutorial, you learned how to:

Step 1: Start a new project.

Step 2: Open a worksheet.

Step 3: Choose an appropriate graph based on your data and intent.

Step 4: Choose an appropriate graph model from a gallery.

Step 6: Use common graph options when creating a graph.

Step 7: Explore graphs with crosshairs and data tips.

Step 8: Investigate individual points and groups of observations with brushing.

Step 9: Use a paneling variable or By variable.

Step 10: Select individual items and groups of items for graphing.

Step 11: Edit basic attributes of graph items.

Step 12: Add graph elements to a graph.

Step 13: Replace graph variables and individual observations without re-creating a graph.

Step 15: Edit structural aspects of graphs.

Step 16: Create a graph layout.

Step 17: Edit Minitab graphs in other applications.

Step 18: Use menu commands to produce command language.

Step 19: Save a project and exit Minitab.

At this point you may want to take a break, or you can continue with Session Two: Entering and Exploring Data.

Overview of Session Two: Entering and Exploring Data

The story

Clones are genetically identical cells descended from the same individual. Researchers have identified a single poplar clone that yields fast-growing, hardy trees. These trees may one day be an alternative energy resource to conventional fuel.

Researchers at The Pennsylvania State University planted Poplar Clone 252 on two different sites: one site was by a creek with rich, well-drained soil, and the other site was on a ridge with dry, sandy soil. They measured the diameter, height, and dry weight of a sample of three-year-

old trees. These researchers want to see if they can predict how much a tree weighs from its diameter and height measurements.

As data analyst for the project, you must determine if diameter and height measurements can be used to reliably predict the yield of wood.

What you will learn

In this session, you will learn how to:

    Enter and edit data

    Compute basic statistics

    Use the calculator to create a new variable

    Create a fitted line plot

    Use simple regression to find the relationship between the two variables

    Rerun your analysis after detection and correction of data errors

Time required

About 30 minutes.

Step 1: Start a New Project Step 2: Open a Worksheet Step 3: Perform ArithmeticStep 4: Enter Data from the Keyboard Step 5: Enter Patterned DataStep 6: Compute Descriptive StatisticsStep 7: Create a Fitted Line PlotStep 8: Perform a Simple RegressionStep 9: Edit the Data Step 10: Run the Regression Again Step 11: Save and Exit

Step 1: Start a New Project

    If you are not already running Minitab, start the program.

    If you have just completed Session One, start a new project: choose File > New, choose Minitab Project, then click OK.

If you have not saved your changes to the previous project, Minitab prompts you to do so.