Creating stem and leaf plots

This tutorial covers the steps for creating stem and leaf plots in StatCrunch. To begin, load the Asking prices for 4-bedroom homes in Bryan-College Station TX data set, which will be used throughout this tutorial. The data set was collected in order to compare four-bedroom homes listed for sale in the two adjoining cities of Bryan, Texas, and College Station, Texas. Using a real estate web site, fifteen homes were randomly selected from four-bedroom homes listed for sale in Bryan, Texas, and fifteen homes were randomly selected from four-bedroom homes listed for sale in College Station, Texas. The price/sqft will be the only column used in this tutorial. The column contains the selling price in dollars of each home divided by its square footage.

Creating a stem and leaf plot

A buyer wants to summarize the price per square foot of homes sold in the Bryan-College Station, Texas, market. To create a stem and leaf plot of the price/sqft variable, choose the Graph > Stem and Leaf menu option. Select the price/sqft column and click Compute!. The resulting stem and leaf plot shown below has four stems with leaves for 28 of the 30 homes in this dataset. The other two homes with prices of $300 and $488 per square foot are displayed on the “High” listing due to the default outlier trimming. The decimal point is shown to be two places to the right of the colon separating the stems from the leaves, so the 9 on the 1 stem represents a value of $190 per square foot. Equivalently, the value of 190 can be obtained by multiplying the leaf value of 9 by the displayed Leaf unit of 10 to get the product of 90 which is then appended to the stem value of 1. This rounded value of 190 corresponds to $187 per square foot value in the second row of the data set. Rounding values is common in stem and leaf plots in order to produce a reasonable display.

Changing the leaf unit

Specifying the Leaf unit option can be used to provide a more or less detailed summary of the data. For example, in the window containing the resulting stem and leaf plot above, choose Options > Edit to reopen the dialog. Under Leaf unit, select 1 and click Compute!. The resulting stem and leaf plot has a smaller leaf unit of 1 which increases the number of stems required to cover the range of the data excluding the outliers. The decimal place is now one place to the right of the colon. The value of $187 per square foot is shown directly in the plot without rounding as a 7 on the 18 stem. This value can be understood as the 7 leaf value multiplied by the Leaf unit of 1 to get the product of 7 which is appended to the stem value of 18.

Changing the handling of outliers

Only extreme outliers are trimmed off the plot by default and placed on separate Low and High listings as necessary. To change how outliers are handled, choose Options > Edit in the window for the previous stem and leaf plot to reopen the dialog window. Under Outlier trimming, select None to not trim any data values from the plot. The Mild and extreme option can be used to make the procedure more aggressive in identifying outliers for trimming. Click Compute! to view the resulting stem and leaf plot shown below. The plot now has stems that cover entire data range with the values of 300 and 488 included in the plot.

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