MINITAB COMMANDS TABLE OF CONTENTS:

Transcription

1 MINITAB COMMANDS Minitab is one of many statistical software packages. This handout contains the commands that you will need throughout this course, and perhaps beyond, and it is meant as a reference for the work you will do on the computer. What is the purpose of using a computer in statistics? Basically, to do the "dirty work." The computer can do the calculations and print pictures much faster than you can do by hand. But, the computer cannot interpret what it does -- that is your job. The commands are available in two forms: pull-down commands and written commands. This handout shows the pull-down commands. See Part I for information on the written commands TABLE OF CONTENTS: Page Part I General Procedures / Commands Part II Working with Data and Data Files Part III Numeric Descriptors Part IV Graphic Descriptors Part V Assessing Normality Part VI Regression Part VII Chi-Square Part VIII Confidence Intervals Part IX Hypothesis Testing Part X Sampling/Probability Part XI Miscellaneous Important Stuff -- Saving, Printing, and More! Part XII How to Find Some Data Sets Part XIII Common Errors / Problems PART I: GENERAL PROCEDURES / COMMANDS Start-up If you are not starting with an existing file, go to Start / Programs / Instructional Applications / Math / Minitab 14 for Windows / Minitab. If you are opening an existing data file, go to My Computer, double click on \\Alexandria\Stats and find the file you need. (See Part XII for more information on finding files.) The Windows Once in Minitab, you will see a menu across the top of the screen and two windows: A session window above and a data window below. The data window is where the data will go, and the session window is where written commands are entered and where some output will appear. To move between windows, just point the mouse arrow in the desired window and click, or go to the Window menu option and pull down to the window of choice. Notice that the active window is the one whose title bar is darker. Click in each window to see how this works -- it is important to know which window is active as some menu options will change. Making the windows smaller/larger: You can make the window larger by clicking on the small middle box in the upper right-hand corner of the window (handy for seeing more output.) To reduce it, repeat the process. Page 1

2 Project versus Worksheet: Minitab uses Projects and Worksheets. The data window is basically a worksheet. You can open several data windows and they become a project. Most data files stored on the computer are worksheet files, so be sure to Open / File worksheet, and not Open / File project. Written Commands As mentioned in the introduction, Minitab has both pull-down and written commands. If you prefer the written commands, they are very easy to use. First, you need to make sure the MTB> prompt is showing in the session window. If it is not, then, with the session window active, go to Editor / Enable Command Language. Once you have the prompt, you can use the typed commands. Most are self-explanatory (HIST for a histogram, BOXP for a boxplot, DESC for descriptive statistics, all followed by a variable location/column number.) For specific syntax, type HELP. Getting help in Minitab Menu Choices: Help / Contents If you need more details on the syntax of a command, just go to Help. PART II: WORKING WITH DATA AND DATA FILES Entering Data into Minitab The most common way to enter data into Minitab is to enter it directly into a column of the worksheet. With the data window open and active, simply put the cursor in the desired cell and enter the data down the column. Hitting return (or enter) should automatically move you down to the next row. The tab key will move you across columns. Be sure to hit return or tab after each entry. (If you see that the cell is outlined, rather than filled in with color, then you haven't hit return.) You can also copy/paste from an Excel spreadsheet, or another Minitab data worksheet. Opening an Existing Data File Menu Choices: File / Open Worksheet A list of files will appear in the left-hand box. You may need to go to a different location to find the file of interest (ask your instructor for help). Once the file is found, double-click on it. Starting with an empty data file If you get into Minitab through Start, you will start with a blank worksheet. If you have a data file open and want to start a new file, go to File / New and choose Minitab Worksheet. Naming a variable It is a very good idea to name variables for easy reference. If you use an existing data file, the variables may already be named. When creating your own data file, you get to choose the names. To do this, go to the data worksheet and enter the name in the box below the column label. Use descriptive names, but keep them short (long ones may not print properly). Minitab Commands Page 2

3 Sorting a Column Menu Choices: Data / Sort Boxes: Sort column(s): By column: Enter the column(s) that will be sorted. Variables can be chosen from the list in the left box by double-clicking. Put the column to be sorted here (seems strange, but is necessary). By default, the sort will be in ascending order. If you want descending order, check that box. Store sorted data in: You have three choices: store the data in a new worksheet, overwrite the old data in the original column, or put the sorted data into a new column of the current worksheet. Just click on your choice. Copying Columns The easiest way to copy a column of data is to highlight it, copy it (right click, or Edit / Copy Cells), and then paste it to the new location. Omitting rows with specific characteristic from copy Sometimes it may be handy to copy just certain values, and omitting others. For example, you may want to create a column for female SAT score -- copying the SAT scores of only the females to a new location. Here is how to do that: Menu Choices: Data / Copy / Columns to Columns Boxes: Copy from columns: Enter the column(s) that will be copied. Variables can be chosen from the list in the left box by double-clicking. Store Copied Data in Columns: You have three choices (click the down arrow at the end of the box): In new worksheet, In following worksheet, In current worksheet. Subset the Data: Click on this box. Next, click on Specify which rows to include or exclude. You can specify which specific rows to include or exclude (if you know the row numbers), or click on Rows that Match and then Condition. In the Condition: box, you can specify what you want. For example, 'Gender' = 1. Then click on OK, and OK, and OK again. Minitab Commands Page 3

4 Formulas and the Data Menu Choices: Calc / Calculator Boxes: Store result in variable: Enter the location (desired column) for the new data. Typically this will be an empty column. Expression: Type your formula in this box. Examples: C1 + C2 5/9 * (C1 + 32) 'Exam1' + 10 PART III: NUMERIC DESCRIPTORS Calculating numeric descriptors Menu Choices: Stat / Basic Statistics / Display Descriptive Statistics Boxes: Variables: Choose the column of data from the list in the left box by double-clicking on it. You can put more than one variable in the Variables: box, just put each one on a separate line. Here is a description of the values calculated: Variable N Variable name/location. The number of values in the data set. N* The number of cells with no values. That is, the number of missing values in the data set. Minitab will code these with an asterisk, "*". If your data set does not have any missing values, then you will not see this label. Mean SE Mean StDev Minimum Q1 Median Q3 Maximum The mean, or average, of the values. The standard error of the mean, which is the StDev divided by the square root of the sample size (N). The standard deviation. The smallest value. The lower, or first, quartile. The median of the data. The upper, or third, quartile. The largest value. Minitab Commands Page 4

5 PART IV: GRAPHIC DESCRIPTORS NOTE: With few exceptions, each graph that you request will appear in its own window. To see a listing of the graphs you have done, go to Window and a list of graphs appears at the bottom. Histogram Menu Choices: Graph / Histogram Boxes: Pick which type of Histogram you want. For most applications, Simple will work. Graph Variables: Choose the column of data from the list in the left box by double-clicking on it. You may do histograms for more than one variable at a time. Each histogram is drawn in its own graph window. Minitab defaults to a midpoint, frequency graph. If there are too many intervals on a graph, some may not be labeled. Changing a particular feature of the graph: Type of Graph: Under the Graph Variables: box, click on Scale. Next, click on Y- Scale Type, and from there you can choose Frequency, Percent, or Density. Change from Midpoint to Endpoint (Cutpoint): Do the initial graph. Once complete, double click anywhere on the bars, and a new edit window will appear. Click on the Binning tab. In this box you can choose Interval Type. Change the number of intervals, or interval spacing: Do the initial graph. Once complete, double click anywhere on the bars, and a new edit window will appear. Click on the Binning tab. Under Interval Definition, you can choose from several options: Automatic (default), Number of Intervals: and specify how many, and Midpoint/Cutpoint positions:. In this last box you have much control over your intervals, but the syntax is very particular: m : n / w where m = left-hand minimum value, n = right-hand maximum value, and w = width of the bars. For example, 10:110/25 would give you bars of 10-35, 35-60, 60-85, and Caution: the max value (n) must be at or higher than the maximum of your bars. For example, if we had put 10:100/25, the max of 110 is outside that range, and we would not get the last interval from NOTE: My general philosophy is that if Minitab cannot label every bar, then you have too many and the graph is hard to interpret. Do what you can to reduce the number of intervals for easier reading. If you have too many graphs open, you may get an error message. If this happens, go to Window and Close All Graphs. Your present graph will then be shown. Minitab Commands Page 5

6 Stem and leaf plot Menu Choices: Graph / Stem-and-leaf Boxes: Graph Variables: Choose the column of data from the list in the left box by double-clicking on it. The stemplot will appear in the session window. You may need to make the window larger to see the whole plot. A note on the output: The left-most column of numbers represents the number of observations on that level or below, until the middle, at which point it counts the number of observations above that level, and including that level. The row with the value in parentheses contains the median. The value in parentheses is the number of values on that line. The leaf unit indicates the position of the leaf in relationship to the stem. Changing stem units Menu Choices: Graph / Stem-and-leaf Boxes: Click in the Increment box. In this box input an increment value, that is, the distance between the smallest value on one line and the smallest value on the next line of the original data. Separating data Boxplot Menu Choices: Graph / Stem-and-leaf Boxes: Check the By: box, and then in the variable location put the location of the separating data. For example, put the variable GENDER in this box and separate stemplots will be drawn for men and women. Menu Choices: Graph / Boxplot Boxes: First choose the type of boxplot you want. For most applications, Simple will work. Graph Variables: Choose the column of data from the list in the left box by double-clicking on it. You can choose several variables at once. The boxplot will appear in its own graph window. Minitab defaults to a vertical boxplot. Any outliers outside 1.5*IQR will be shown with an asterisk "*". Changing to a horizontal boxplot Menu Choices: Graph / Boxplot / Scale Boxes: Click on Transpose value and category scales Click on OK and OK. Minitab Commands Page 6

7 Side-by-side boxplots Menu Choices: Graph / Boxplot Boxes: Choose the With Groups option instead of Simple. Graph Variables: Choose your main variable. Categorical variables for grouping Choose the grouping variable (such as Gender). Click on OK and OK. PART V: ASSESSING NORMALITY Drawing a normal probability plot Go to Graph / Probability Plot. Choose Single, then put your desired variable in the Graph Variables: box. Hit OK. PART VI: REGRESSION Scatterplot with Regression Line and r 2 Menu Choices: Stat / Regression / Fitted Line Plot Boxes: Response (Y): Put the variable on the y-axis (response variable) here. The variables are listed in the box on the left and can be chosen by double-clicking. Predictor (X): Put the variable on the x-axis (explanatory, or predictor, variable) here. The variables are listed in the box on the left and can be chosen by double-clicking. When appropriate boxes are filled, click on OK. It will take a few seconds while the macro runs, and then the plot with line, the regression equation, and the r 2 value will be given, all in a graph window. In the session window, you will get a list of regression output that includes the regression equation and r 2. Residuals Plot First, in order to do a residuals plot, the residuals themselves must be calculated. This is best done when you do the Fitted Line Plot (above). After entering your x and y variables, click on the Storage box and then click on the Residuals box. Next, click on the Graphs box. At the bottom in the Residuals versus the variables: box, click in the box and then choose your x-variable from the list at left. Click on OK, and OK. You will get the fitted line plot as well as the residuals plot! Minitab Commands Page 7

8 Scatterplot only Menu Choices: Graph / Scatterplot Boxes: Choose Simple. Graph Variables: In the Y variables box put the location of the y-axis data and in the X variables box put the location of the x-axis data. The scatterplot is drawn in its own graph window. Regression calculations only Menu Choices: Stat / Regression / Regression Boxes: Response: Put the variable on the y-axis (response variable) here. The variables are listed in the box on the left and can be chosen by double-clicking. Predictors: Put the variable on the x-axis (explanatory, or predictor, variable) here. The variables are listed in the box on the left and can be chosen by double-clicking. The output will print in the session window. There is a lot of information that you will not need, but you should be able to find the regression equation and r 2, and a listing of Unusual Observations. Correlation Coefficient (r) Menu Choices: Stat / Basic Statistics / Correlation Boxes: Variables: Choose the y- and x-variables. The variables are listed in the box on the left and can be chosen by double-clicking on them. The correlation coefficient (r) will be printed in the session window. PART VII: CHI-SQUARE Chi-Square calculations Menu Choices: Stat / Tables / Chi-Square Test Boxes: Columns containing the table: Choose the column locations containing the table -- the columns with the observed values. Minitab Commands Page 8

9 PART VIII: CONFIDENCE INTERVALS Calculating a t-interval Menu Choices: Stat / Basic Statistics / 1-Sample t Boxes: If your data is in a column(s), then click on the Samples in columns: box. Click in the box under this heading, choose your variable. If you just have the statistics, then click the Summarized data box and fill in the requested information. Minitab defaults to a 95% confidence interval. Options and change the level. If you want to change this, go to Click OK. PART IX: HYPOTHESIS TESTING Calculating a t-test Menu Choices: Stat / Basic Statistics / 1-Sample t Boxes: [Note: This is the same location as for a Confidence Interval.] If your data is in a column(s), then click on the Samples in columns: box. Click in the box under this heading, choose your variable. If you just have the statistics, then click the Summarized data box and fill in the requested information. For a hypothesis test, you must put a value in the Test mean: box. Minitab defaults to a two-sided test (not equal). If you want to change that, go to Options and then under Alternative: you can make your choice. Click OK. Two sample t-test Menu Choices: Stat / Basic Statistics / 2-Sample t Note: Minitab will do the two-sample hypothesis test, as well as the confidence interval for differences. Boxes: You have three choices: (1) Your data is in one variable (click Samples in one column) that will need subscripts (such as comparing heights of men and women where GENDER will be the subscript), (2) Your data is in separate variables (click Samples in different columns, such as comparing high school GPA to UPS GPA), and (3) You have Summarized data. Whichever choice is checked, fill in the appropriate boxes. Minitab defaults to a 95% confidence interval, and the not equal alternative hypothesis. To change either of these, go to Options. Click OK. Minitab Commands Page 9

10 PART X: SAMPLING/PROBABILITY Get a Random Sample Menu Choices: Calc / Random Data / Sample from Columns Boxes: Sample Enter the size of the sample. rows from column(s): Put the name/location (column) from which the sample will be taken. The variables are listed in the box on the left (click on the rows from columns(s) box to see the list). Store samples in: Put the location of the sample (make sure this is an empty column! Generating Random Numbers Menu Choices: Calc / Random Data / type of generator Choose the type of distribution from which you want the numbers generated. There are many choices. Here are some you might find useful. Normal Uniform Integer Generates values having the normal distribution. You need to give a mean and standard deviation. Generates equally likely values in a given range. For instance, any number between 0 and 1, each value being equally likely. This is different from the Integer generator in that decimal values are included up to eight decimal places. Generates random integers between a starting value and ending value -- all values equally likely. Boxes: Generate... rows of data In this box put the size of the sample of random numbers desired. Store in column(s): In this box, put the location of the random data. You may enter more than one column. Miscellaneous boxes Each distribution will require more information (range of integers, mean and standard deviation, etc.) Click OK. Minitab Commands Page 10

11 PART XI: MISCELLANEOUS IMPORTANT STUFF Saving, Printing, and More On Saving: The data files you access, for the most part, are on the hard drive on the computer. There shouldn't be any overwhelming reason for you to save your work when using these files -- you do it and print it out, or copy/paste into Word, and you're done. In fact, if you are using an existing data file, the system will not even let you save changes on the system disk. To save a data file: Go to File and then Save Current Worksheet As. With this particular sequence you can change the location of the file to be saved. Click on the arrow on the right of the Save in: box and you can pick the location where you want the data file saved. If you go to File / Save, you will probably run into an error -- you should not be able to make changes to an existing data file. To save the graph windows: With the graph open and current, go to File and Save Window As. You can then specify a location and name for the graph. This will save the currently open graph. These graphs can be accessed only through Minitab. The best way to save your work is to cut/paste it into a Word document, and then save that document (see next page). IN GENERAL: You can save to a disk, or you can save to your file on the network. When you click on that arrow in the Save in: box, you will find a folder called username on Alexandria. This is your hard-drive folder and you can store files here easily. Doing this will also make that file available from any other computer you log onto the network from. On Printing: 1. If you have a graph open and as the current window, you can go to File / Print Graph and it will print on a full page. 2. If you want to print something that is in the Session Window, then first highlight it using the mouse, and then go to File / Print Session Window. Only the highlighted portion will be printed. 3. The better way to print, and save on paper and the true cut and paste, is to copy your output to a Word document, and then print from there. See next page. On Using a Word Processor with Minitab: Copying a graph: With the graph window open and current, go to Edit / Copy Graph. Then go to your word processor and go to Edit / Paste Special and then choose Picture or Bitmap. With this method you will be able to re-size your graphs. If you do a simple Edit / Paste, then you will not be able to re-size the graphs. Copying the Session window: Use the mouse to highlight the output in the session window. Then go to Edit / Copy. Next go to your word processor and to Edit / Paste. You will probably need to change the font style to Courier or Monaco so that the output lines up properly. You may also need to change the font size, especially on the Descriptive Statistics, so that you avoid wraparound. Minitab Commands Page 11

12 PART XII: HOW TO FIND SOME DATA SETS To find: stats on Alexandria First single left click on My Computer, followed by a single right click. Then pull down to Map Network Drive and in the Path box type \\Alexandria\stats. (You should only have to map the network drive once -- next time you log on, it should be there.) To find: Minitab Go to Start / Programs / Instructional Applications / Math / Minitab 14 for Windows / Minitab 14. To find: M160Fxx or M160Sxx (where xx = current year) Double click on My Computer (upper left). Look for stats on 'Alexandria'. If it is there, double click on it, then the Paradise folder, followed by the Surveys folder. Find the appropriate survey file and double click. To find: Class Notes exercise data Go to stats on Alexandria, then go to the A. Paradise folder. In this folder you will find a folder called Class Notes Data. The data is in this folder by Unit and exercise number. Minitab Commands Page 12