Introduction to JMP and Describing a Categorical Variable
In this example we will be working data which comes from a study of corncrakes (crex crex) where the goal was to get bird counts for a number of geographic areas in the UK (see map below). It is rare bird with remaining strongholds in north-west Scotland and Ireland. The data gathered by researchers consists of a geographic area that was sampled and an estimate of the number of corncrakes inhabiting that area.
Entering the Data
From the JMP Starter window click the New Data Table button. A new spreadsheet will appear with only one column (labeled Column 1). We need two columns in our spreadsheet to enter this table, one for the geographic area and one for the number of corncrakes inhabiting the area. To add columns to a spreadsheet simply double click to the right of the first column. Each time you double click to the right of the rightmost column another column will be added. Here we only need one additional column so we will double click once to the right of the first column. All columns in JMP are set to receive only numeric input by default. Because the geographic area information is non-numeric or categorical data we need to change the data type for this column. To do this right-click at the top of Column 1 and then selectColumn Infofrom the pop-up menu. Awindow will open which will allow you to change the name of the column, the data type, the modeling type, the column width etc. For the name we will use Geographic Area and for the data type we will use Character because we will be entering the name of the area sampled. Also because some of the area names are quite lengthy, we will increase the Field Width to 30. Notice that by changing the data type to character causes the modeling type to become nominal (i.e. categorical/qualitative).
For the second column all that is required is a name change. This can be done using the process described above, or by simple clicking on the current column name, Column 2, and changing the name to # of Corncrakes. These counts are actually frequencies and we wish the computer to interpret them as such. To do this we will use the role assignment pop-up menu to change this column’s role to that of a frequency count. The Preselect Role menu is accessed by right-clicking at the top of a column. From this pull-out menu select Freq to change this column’s role to frequency/count.
We are now ready to input the information contained in the table but we first need to add 10 rows to our spreadsheet. To do this select Add Rows from the Rows pull down menu and change the number of rows to add to 10, which the is the number of geographic areas considered in this study. Use the mouse, the return key, and/or the arrow keys to move about the spreadsheet and enter the data. When you are finished your spreadsheet should look like:
Note: Rows will automatically be added each time you hit enter when you are entering data in the last row.
Descriptive Methods for Categorical/Qualitative Data
Mosaic Plots and Frequency Tables
To obtain a mosaic plot and frequency distribution table for these data select Distribution from the Analyze pull down menu and place Geographic Area in the right-hand box by double-clicking on it. Below is the resulting bar graph and mosaic plot for these data.
A relative frequency or probability axis has been added by selecting Prob Axis from the Histogram Options pull-out menu. The Histogram Options menu is accessed from the pull down menu (red triangle) located next to the variable name, in this cases Geographic Area. The Show Percents option has been selected from the Histogram Options menu as well. Also a mosaic plot (rectangular pie chart) has been added by selecting that option from the Geographic Area pull-down menu.
Below is the resulting frequency table for these data:
The Count column contains the frequencies and the Prob column contains the relative frequencies.
All output in JMP can be copied and pasted into Microsoft Word by selecting Edit > Copy in JMP and pasting in Word.
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