Massachusetts Institute of Technology
Department of Urban Studies and Planning

11.220 Quantitative Reasoning and Statistical Methods for Planning

SPSS

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SPSS > Boxplot

Boxplot is a summary plot of your dataset, graphically depicting the median, quartiles, and extreme values. The box represents the interquartile (IQ) range which contains the middle 50% of the records. The whiskers are lines that extend from the upper and lower edge of the box to the highest and lowest values which are no greater than 1.5 times the IQ range. A line across the box indicates the median. Outliers are cases with values between 1.5 and 3 times the IQ range, i.e., beyond the whiskers. Extremes are cases with values more than 3 times the IQ range. The mean is indicated by an x, shown just above the median.

1. To create a boxplot in SPSS go to Graphs->Boxplot.

2. Click the "Define" button.

3. Select "incws" in the left pane, then click the arrow button next to "Variable:." Likewise, select "male" in the left pane, then click the arrow button next to "Category Axis:." Click OK.

4. An output window pops up and the following graph appears. The vertical axis shows the dollar amount of the annual wage/salary variable and the horizontal axis shows gender (0 for female, 1 for male). The graph shows that the range of wage/salaries for men is greater than for women and the IQ range (middle 50% of records) is higher on the wage scale in men than in women. Additionally, the placement of the median value (middle value in the distribution) for women indicates that half of the IQ range values are in a very small lower portion of the IQ range. The graph suggests that males tend to earn more money than females and there may be some degree of wage/salary discrepancy between male and female.

Created by Myounggu Kang on January 25, 2004. Edited by Rhonda Ryznar on January 20, 2005.