CHAPTER 1



CHAPTER 1

AN INTRODUCTION TO MINITAB

Organization of this manual

For each chapter of the textbook, except Chapter 3, you will find a chapter in this manual containing instructions on how to solve selected examples using Minitab. The instructions are described along with the corresponding Minitab screen and all the necessary input to obtain the results displayed in the book. Color has been used to help you navigate through this manual making possible an easy identification of what you are looking for.

Color red is used for references to the book, for example ‘Example 13.1.’

Color green is used to refer to variables, data sets, and files, for example ‘signdist.mtw.’

Color blue is used to write Minitab commands or refer to the main menu bar. For example, the File>Open Worksheet notation refers to the option Open Worksheet inside File of the main menu bar.

Content of Chapter 1

1. The Minitab environment: Worksheet and Session windows

2. Minitab main menu

3. The File menu

4. Opening data files

5. Saving data files and projects

6. Printing data

7. Using the Menu vs. typing commands, the MTB> prompt

8. Looking for help

9. The Stat menu lists the statistical methods available

10. The Graph menu lists the graphs available

11. Choosing the window you want to use

12. The Calc menu:

• Calculating new variables

• Summarizing the data in columns or rows

• Creating a variable with values that follow a pattern

• Selecting random samples

• Working with probability

13. The Data menu

• Copying columns

• Stacking and Unstacking columns

• Ordering the values of a variable

• Erasing rows or columns

• Changing the values of a variable according to a code

• Changing a column from text to numeric and vice versa

• Displaying part of the worksheet in the Session window

• Other features for the more advanced user

14. Editing in Minitab

15. Copying Minitab graphs and output into a document to prepare reports.

1. The Minitab environment

To start Minitab, click on the Minitab icon [pic]. The screen will look as follows:

[pic]By default the screen of Minitab is divided into two parts, the Session window (where results will be displayed and commands can be typed) and the Worksheet window (where data is entered and results can be stored). You can make either of those windows appear full screen by clicking on the πsymbol that is in the upper right corner of each window. If you click π of the Worksheet or data window, the screen will look like the following:

[pic]If you click πof the Session window, the screen will look like this:

[pic]

To return to the split window environment, simply click on the symbol formed by the two overlaid squares between the symbols _ and X in the upper right corner of the Minitab window. If you want to minimize a window, click on the symbol _ and if you want to close a window then click on X.

2. The Minitab Main Menu

The options in Minitab’s main menu bar are:

File Edit Data Calc Stat Graph Editor Tools Window Help

Each one of these main menu options has its own submenu of options-an overview of them will be given in this chapter. Minitab has many options and capabilities; we will focus only on those that are necessary to work with the material presented in the book.

3. The File menu

[pic]The options of the File menu that will be used in this manual are:

a) New: To create a new empty worksheet.

b) Open Project: This option opens worksheet(s), output, and graph(s) that were previously saved in a project file.

c) Open Worksheet: This option opens a data file (we will expand on this option in the next section).

d) Save Current Worksheet: To save the worksheet you are working with after you have made some changes.

e) Save Current Worksheet as: To save the worksheet with another name or as a file of a different type.

f) Exit: To close Minitab. This is equivalent to closing the program by clicking on the X symbol.

4. Opening data files

Selecting Open Worksheet from the File menu will open the window shown below.

You now want to indicate, in the Look in dialog box, which folder contains the file that is to be opened. On the CD that comes with the textbook, there is a folder called DataSets where the data for the examples are located. Since we are working with Minitab, you can select either the folder MTP or the folder MTW. Both folders contain data files that can be used in Minitab. We will be working with the MTW folder since the standard data files in Minitab have the extension mtw.

[pic]

The list of all the available data files will appear. Make sure that the option Minitab (*.mtw) appears in the dialog box of Files of type; if not, the list of files with extension mtw will not appear in the window.

[pic]To select the file to be opened in the Worksheet window, either double-click on the name of the file or enter the name of the file in the File name dialog box. For example, double-clicking on the file handheight produces the following window.

[pic]

Click OK to open the file in the Worksheet window. Each row contains the data for one individual and each column contains the data of one variable. Each column is numbered and referenced by C1, C2, C3, etc. or by names such as Sex, Height, and Handspan. A letter T next to the name of the column indicates that the variable in that column is categorical and nonnumeric (e.g., Sex (C1-T) is categorical and the values of the variable are M or F).

Minitab can also work with constants K1, K2, etc. which contain a single value. Storing a value in a constant can be useful when the value is used in other computations. Finally, Minitab can also work with matrices M1, M2, etc. which are arrays of values. This is useful for more advanced statistical methods.

You can have several worksheets open simultaneously. To navigate between worksheets, click on the worksheet that you want to become active, or it can be selected from within the WINDOW option of the main menu.

[pic]Alternative. Instead of working with MTW files you could also work with MTP files. Select FILE>Open Worksheet from the main menu, and once again choose the DataSets folder in the Look in dialog box, but this time select MTP.

[pic]

To get the list of the data files with the extension mtp make sure that in the Files of type dialog box, the option Minitab Portable(*.mtp) appears.

[pic]Minitab portable files look similar to the MTW files, but they occupy less physical space on the disk, and they can be transferred from one type of computer (IBM compatible) to another (Macintosh).

Note. Other types of files (e.g. Excel, Text) can be opened in Minitab, which is indicated by the options that appear in the Files of type dialog box.

5. Saving data files and projects

If you make changes to a data file and want to save it with the same name use

File>Save Current Worksheet

If you want to save the data file under a different name, use

File>Save Current Worksheet as

It is understood that the worksheet that gets saved is the worksheet that is currently active.

If you use Save Project instead of Save Current Worksheet, not only will the worksheet with the data be saved, but all the graphs and results you have obtained during that session will be saved as well.

To close a worksheet simply click on the X in the upper right corner of the worksheet or use the option File>Close Worksheet

6. Printing Data

Printing the whole worksheet on paper

You can print the active worksheet by choosing from the File menu the option Print Worksheet.

Printing part or all of the worksheet in the Session window

If you want to display data in the Session window then select Data> Display Data from the menu and enter the columns (e.g. C1-C3) in the dialog box. If you want to insert the data in a text document you are preparing with another software (e.g. Microsoft Word) first use the data of the worksheet in the Session window:

Row Sex Height HandSpan

1 Female 68.00 21.5

2 Male 71.00 23.5

3 Male 73.00 22.5

4 Female 64.00 18.0

(continues)

You can use the mouse to highlight the text in the Session window and then use Edit>Copy, go to Word (or any other word processor software you are using) and paste the text there. Note that the data can also be displayed in the Session window by typing MTB> print c1-c3

7. Using the Menu vs. typing commands, the MTB> prompt

There are two ways of getting Minitab to do a task, one is by using options of the menu bar; the second is by typing a command in the Session window. The task that we want to perform will dictate which method is simpler and faster. Most of the time it is easier to use the menu but there are some tasks that require some very simple commands, in which case it is faster to type them. In this manual we will use the menu but the command line in the Session window will also be used when using it is simpler and faster. In order to type a command, we need to have the MTB> prompt displayed in the Session window. When we do not see the MTB> prompt in the Session window, place the cursor in the Session window and from the menu select Editor and click on Enable Commands. The MTB> prompt will appear and then you can type in the commands you want.

[pic]While the MTB> prompt appears in the Session window, Minitab will automatically show the commands used to perform each task we accomplish by using the menu. This is a good way of learning about Minitab commands.

[pic]

8. Looking for Help

All Minitab instructions needed to replicate the results of the selected examples from the textbook are included in this manual. However, if you want to find instructions for other tasks performed by Minitab, the program has a help file in the menu. Select Help>Help.

[pic]In the dialog box you can type the name of your request. For example, if outlier is the subject of interest, type outlier in the dialog box and a list of all the topics related to outliers will appear. Select the index entry closest to your topic of interest.

[pic]

9. The Stat menu: Statistical Methods available in Minitab.

The Stat option in the menu contains the statistical methods that can be applied using Minitab. Selecting the Stat option of the menu produces the following options:

[pic]From the Stat menu list, the items used to work the examples in the textbook are:

Basic Statistics

Regression

ANOVA (Analysis of Variance)

Tables

Nonparametrics

EDA (Exploratory Data Analysis)

10. The Graph menu: graphs available in Minitab.

Minitab can produce many different graphs (e.g. bar graph, pie chart, stem-and-leaf, histogram, etc.) and they are located under the Graph option of the main menu. All graphs appear in the Graph window, except for the Character Graphs, which appear in the Session window. The graphs that appear in the Graph window are of high-resolution and can be edited, saved, and printed.

[pic]The graphs that are used in this manual are the following.

Scatterplot produces the scatterplot for two variables. The plot is useful in looking for relationships between quantitative variables.

Histograms, dotplots, and stem-and-leaf are the most commonly used graphs to display the shape of a quantitative variable.

Pie chart is used with a categorical variable.

Bar chart can be used to plot the frequencies of categorical variables.

Probability Distribution Plot can be used to view area under a probability distribution.

Boxplot can be used to compare distributions.

For most graphs there are additional submenus (e.g. options, annotation) in the window of the Graph option where specifications can be given to embellish or add more information to the graph. For example, the observations of the variables ‘limit’ and ‘death’ contained in the file speedlimit.mtw, can be plotted in a very simple scatterplot (see first plot below). Additional items such as, a title, colors and labels for the data can be added to the scatterplot (see second plot below).

[pic]

[pic]

11. Choosing the window you want to look at

While working with Minitab you might reach a point in which you have several worksheets open and have done several plots. The screen can get a little messy. In order to select the item you want to look at, click on Window from the main menu and select the one you are interested in from the list of the current items shown.

12. The Calc menu

This option of the main menu has several features to perform intermediate or auxiliary calculations useful in data analysis such as selecting random samples to do simulations and to work with probability. From the Calc menu we will comment on the topics that are the most useful within the context of the textbook.

[pic]Calculator can perform transformations (e.g. logarithms, square root, etc.) of the values of a column and store the result in another column. It is also used to do calculations involving several columns, such as calculating the average of two columns. The idea is to create a new column as a function of one or more existing columns. In addition, you can Assign as a formula to a column or stored constant; columns update when data in the worksheets change.

[pic]

Column statistics is used to calculate summaries (e.g. sum, number of observations, mean etc.) of the data values in one column. The different summaries that can be calculated are displayed below. The calculated value can be stored as a constant. Constants have names such as K1, K2 etc., but we can give them any proper name that will help us remember their meaning.

[pic]

Row Statistics can be useful in providing summaries for the values of a row(s). The options are:

[pic]

Make patterned data. This option is a quick and easy way to create a column of numbers, such as 1,2,3,…… instead of typing them one by one. This option will be used in the chapters related to probability.

[pic]Random data is used to select random samples from existing columns. One can also simulate a random sample of observations from a given distribution.

Probability Distributions is a useful option for the computation of probabilities for many distributions such as the normal, binomial, t, and chi-square.

13. The Data menu

Clicking on Data of the menu we find some useful options such as:

Copy, used to copy the content of one column into a new column

Unstack columns, to separate the contents of a column into one or more new columns. For example, a column contains the heights of men and women and we want to have two separate columns, one for men and one for women. To perform this task it is necessary to have another column besides the one of the heights, that other ‘indicator’ column is a column that contains the information about the gender for each one of the individuals.

Stack columns, to merge two or more columns of values into one column. For example, if there is a column with the weights of women and another with the weights of men, a new column can be created with the weights of all individuals regardless of gender.

Sort, used to order the values of one or more columns according to the values of a given column either in ascending or descending order. For example, if we have the last name, first name, and exam scores for a group of students, the list of students can be ordered according to the exam scores.

Delete rows, used to eliminate one or more rows of the worksheet.

Erase variables, used to eliminate one or more columns of the worksheet.

Code, used to change the values of a variable using a certain type of coding. For example, if there is a variable X that ranges from 0 to 10, but our interest is only whether the value X is above 3. A new column containing a new variable Y can be created where Y is the value 0 (or NO) if X[pic]3 or Y is the value 1 (or YES) if X>3. In this example, a new column of values have been created, but note that the results could have been stored in the variable X; replacing the original values of X with the new values.

Change data type. In Minitab, the data in each column are either numeric or categorical. We might want to switch from one to the other. This is particularly useful when we make an error when typing new data, such as typing a letter in a numerical column. Minitab will instantly convert the column to a text column and we might want to reverse that action.

Display data, used to display the observations of all or part of the worksheet in the Session window.

[pic]For more advanced users working with data from a survey, there are more useful tasks that can be performed using options of the Data menu, such as:

Subset worksheet, used if one wants to analyze only part of the individuals (rows) without deleting the other ones.

Split worksheet, used if one wants to split the set of observations (rows) into two or more new worksheets (e.g. You may want to split a worksheet that contains information on men and women into two worksheets, one for men and one for women.)

Merge worksheet, to create one single worksheet out of two or more worksheets.

14. Editing in Minitab

In Minitab, the usual options of copy, cut, clear, paste and delete (after highlighting the selected portion using the mouse) are also available in the Edit option of the menu. These editing options are available for both the cells of the worksheet and the output.

Note that the option Output Editable in the Editor menu must have a check mark before trying to edit the output in the Session window.

15. Copying Minitab graphs and output into a document to prepare reports.

You can select (using the mouse) and copy the output in the Session window then paste it into a document prepared by another program (e.g. Microsoft Word). After highlighting your selection, to make a copy of the selection you can either use the right mouse button or the option Copy in the Edit menu. This is particularly useful in preparing reports.

Graphs can also be copied into a word document by first clicking the right mouse button, when the pointer is on the graph, and selecting Copy Graph or by using the menu option Edit>Copy Graph. If you are using Microsoft Word to prepare the document, use the Edit>Paste Special to paste the graph into the document. For more control of the placement of the graph in the document, an option to consider first is to create a table that is 1 by 1 or larger in MS Word and then paste the graph into the cell of the table.

After the graph(s) have been pasted as Minitab Graph Object, you can change its size by placing the mouse pointer on a corner of the picture and then stretch/compress it as much as you want.

[pic]

Chapter 1 is an overview of Minitab capabilities useful in the context of solving the exercises and examples of the textbook. In the following chapters you will find specific Minitab instructions to replicate the results presented in many of the examples.

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