Introduction to MINITAB in the Saint Mary's Microcomputer Lab

IV. Graphs

Minitab produces two general types of graphs: the standard graphing commands produce High resolution graphs which use graphics drawing methods and appear in special graph windows (but are not attached to the session file - so they have to be saved and printed separately). You can also get somewhat clunky graphs drawn in the Session window (so they print with the session window) using the Character graphs commands (There is a Character graphs submenu at the bottom of the Graph menu). For information on saving the results, see VI. Saving and printing your work

A: The graphs you may want

Histogram: Use Graph>Histogram. Select the variable(s) by double-clicking in the list. The Annotation button will let you write a title for your histogram. The Options button will let you choose 1. the type of histogram (frequency, relative frequency, cumulative, etc.) 2. Whether the bars are labeled by the class midpoints or cutpoints (class limits) 3. The number of classes or the values for either the midpoints or the cutpoints.[Automatic is the default - the program decides]. Any options you set will remain in place for your session (until you Exit Minitab) unless you change them.

Boxplots: Use Graph>Boxplot. The variable to be plotted goes in Y - so the boxplot will be drawn vertically.. If several side-by-side boxplots (for categories) are desired, the values must be stacked in one column, with a second column containing an index identifying the category from which each value comes.

Normal probability plots: Use Graph>Probability Plot... Select the variable, leave the distribution choice as "Normal".The graph will print with "confidence bands" around the 45-degree line. The data values are on the horizontal axis, the percentile labels (rather than z-values) are on the vertical axis (the spacing is arranged to match equal spacing of z-values).The "percentile" table in the Session window shows where the percentiles would be if the data were normally distributed (with the same mean and standard deviation)

Scatter plot:Use Graph>Plot. You must specify the variables for the vertical (response - y) and horizontal (predictor/explanatory - x) axes by moving the cursor to the correct box (with mouse or tab key) and either typing them in or double-clicking on your choices. With the Annotation button you can give a title to the graph and label the axes, with the Frame button you can set the minimum and maximum x and y values for the axes. The Data display options are used for having different groups of points (data for males & females, perhaps) plotted with different symbols: You must have a column that indicates which group each row comes from (might be column full of M's and F's) .Under "Data display" (in the window) use the popup menus to select Display symbol for each group and enter the name of the column with the group variable (the column of M's & F's - or whatever) under "Group Variables". To select the symbols to use, click on the "Edit Attributes" button - you can choose a symbol (circle, plus, star, etc.) for each group in the popup menu of the window that opens.

Stemplot: Use Graph>Stem-and-Leaf . Select the variable(s) by double-clicking in the list at the left of the window. You can set the increment at 5 to get a split stemplot. In the display, an extra column at the left of the display shows the "depth" of the class - the distance (number of items) from the nearer end (large or small) of the data. The row with the median is marked in parentheses.This is really a character graph - it shows up in the Session window - it's on the main graph menu because people use it a lot.

Dotplot: Use Graph>Dotplot. Choose the variable(s) you want by double-clicking on them in the window. If you are plotting several variables and want them all on the same scale (so you can compare them) choose the "Each column constitutes a group" option. Use the "Title" box to title the graph.

B: Character Graphs:

Choose the graph using the command as shown (the "Character graphs" option is at the bottom of the "Graph" menu list). The Stem-and-leaf and Dotplot commands always produce character graphs.

(Grouped) Frequency distribution: Use Graph>Character Graphs>Histogram. Select the variable(s) by double-clicking in the list. The Interval is the class width (class interval), the First midpoint is the class midpoint of the first class. The resulting printout does not show class limits but does show class midpoints and frequencies.

Ungrouped frequency distribution (counts for different values): Use Stat>Tables>Tally Choose the columns you want by double-clicking on them. You can choose counts, percentages, etc. by clicking on the boxes next to the options you want.

Scatter plot: Use Graph>Character Graphs>Scatter Plot. You must specify the variables for the vertical (y) and horizontal axes by moving the cursor to the correct box (with mouse or tab key) and either typing them in or double-clicking on your choices. With the " scale " option you can set the graph scale, with the " annotate " you can label the graph and axes.

Last modified 8/10/2000


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