Graphs with SPSS

Topics we will cover today Fundamental Features of Graphs Types of graph How to Choose appropriate graph Various Graphics in SPSS Pie chart Bar chart Histogram Frequency Polygon Line chart

Graphs Graphs are pictorial representations of numerical data. “A picture is worth a thousand t-tests.” A graph is the best way to communicate numerical information to people. Always graph data if you want to understand them or explain to others.

Graphical displays should: Easily convey characteristics of the data. Present many numbers in a small space. Make large datasets coherent. Encourage the eye to compare different sections of data. Be closely integrated with the statistical and verbal descriptions of the dataset. Be clearly labeled for easy understanding.

Fundamental Features of Graphs All graphs have two, clearly-labeled axes that are drawn at a right angle. The horizontal axis is the abscissa, or X-axis. The vertical axis is the ordinate, or Y-axis. The independent variable is plotted on the X-axis, and the dependent variable is plotted on the Y-axis. The Y-axis must be a numerical scale. A graph contains only information related to the data and information that is necessary to interpret the graph.

Types of graph: There are many types, and no laws stopping you from inventing a new format. My aim for today is to show you the theory and practice of the commoner types of graph. Then I will get you used to plotting them in your head to model the behaviour of different patterns within your data (rest assured that this s very quick and easy). Then we head for the PCs to do them ourselves.

Choosing appropriate graph Graphs are selected based on the scale and continuity of measurement. Scale of Measurement Continuity of Measurement Type of Graph  Interval/Ratio Continuous Line Graph Interval/Ratio Histogram Ordinal  Discontinuous Bar Chart Nominal  

The Bar Graph The Bar Graph: a form of graph that uses rectangles, or bars, to represent nominal categories. The bars do not touch. The nominal categories are plotted on the X-axis. The Y-axis of the bar chart may represent frequencies, percentages, or descriptive statistics such as the mean.

The bar can denote: Frequency (number of observations having that categorical value). Fraction (proportion of total observations having that categorical value). Cumulative Frequency (each bar represents a total number of patients who falls in the category or categories in lower orders. ) Mean (or other summary measures) of other variable for the category

How to obtain Bar Graph in SPSS using Graph Option (1) 1. Select File ➪ Open ➪ Data and open the Employee data.sav file. The file is in the SPSS installation directory. 2. Choose Graphs ➪ Chart Builder. 3. In the Choose From list, select Bar. 4. Select the first graph image (the one with the Simple Bar tooltip) and drag it to the panel at the top of the window.

How to obtain Bar Graph in SPSS using Graph Option (2): 5. In the Variables list, select Education Level and drag it to the X-Axis rectangle. 6. In the Variables list, select Current Salary and drag it to the Count rectangle. The label changes from Y-Axis to Mean to indicate the type of variable that will now be applied to that axis. 7. Click the OK button.

Stacked bar charts A stacked bar chart displays multiple values of a variable for each value of a categorical variable. But it does it by stacking them instead of placing them side by side.

How to obtain Stacked Bar Graph in SPSS using Graph Option (1) 1. Choose File ➪ Open ➪ Data and open the Cars.sav file. The file is in the SPSS installation directory. 2. Choose Graphs ➪Chart Builder. 3. In the Choose From list, select Bar. 4. Select the third graph image (the one with the Stacked Bar tooltip) and drag it to the panel at the top of the window.

How to obtain Stacked Bar Graph in SPSS using Graph Option (2) 5. In the Variables list, do the following: a. Select Model Year and drag it to the X-Axis rectangle. b. Select Horsepower and drag it to the Count rectangle. The rectangle was originally labeled Y-Axis. The label changed to help you understand the type of variable that needs to be placed there. c. Select Number of Cylinders and drag it to the rectangle in the upper-right corner, the one now labeled Stack. 6. Click the OK button.

Histograms Histogram: a form of bar graph used to represent information when the X-scale is in interval. From it, we can see whether the cases cluster around a central value. We can also see whether large and small values are equally likely and whether there are values far removed from the rest.

How to obtain Histogram in SPSS . Choose File ➪ Open ➪ Data and open the demo.sav file. The file is in the SPSS installation directory. ► from the menus choose:  Graphs    Chart Builder... ► Click Histogram if it is not selected. Click Simple Histogram. ► Now drag Household Income from the Variables list to the x axis drop zone. ► Click Ok.

Frequency polygons A frequency polygon is a histogram that looks like an area graph (described in the next section) — but it is a histogram, so it works differently. It represents the number of items that appear within a range.

How to obtain Frequency polygons in SPSS 1. Choose File ➪ Open ➪ Data and open the Cars.sav file. 2. Choose Graphs ➪ Chart Builder. 3. In the Choose From list, select Histogram. 4. Drag the third graph diagram (the one with the Frequency Polygon tooltip) to the panel at the top of the window.

How to obtain Frequency polygons in SPSS 5. In the Variables list, do the following: a. Select the Model Year variable and drag it to the X-Axis rectangle in the panel. b. Select Miles Per Gallon and drag it to the Mean rectangle. This is the rectangle that was originally called the Y-Axis. 6. Click the OK button.

The Line Graph Line Graph: a form of graph in which a continuous line indicates the frequency or statistic presented along the Y-axis

Charts with multiple lines A line chart works well as a visual summary of categorical values. Line charts are also useful for displaying timelines because they demonstrate up and down trends so well. Line graphs are popular because they’re easy to read.

How to obtain multiple line in SPSS 1. Choose File ➪Open ➪ Data and open the Cars.sav file. The file is in the SPSS installation directory. 2. Choose Graphs ➪ Chart Builder. 3. In the Choose From list, select Line to specify the general type of graph to be constructed. 4. To specify that this graph should contain multiple lines, select the second diagram (the one with the Multiple Line tooltip) and drag it to the panel at the top. The Element Properties dialog box pops up, but you can close it because the default values work fine.

How to obtain multiple line in SPSS 5. In the Variables list, select Number of Cylinders and drag it to the rectangle named X-Axis in the diagram. 6. In the Variables list, select Engine Displacement and drag it to the Y-Axis rectangle in the panel at the top. The word Mean is added to the annotation because the values displayed on this axis will be the mean values of the engine displacement. In the Variables list, select Horsepower and drag it to the Y-axis also. 8. When the Create Summary Group window appears, telling you that SPSS is combining the two variables along the Y-axis, click the OK button. 9. Click the OK button.

Pie Diagram The Pie Chart is an alternative to the Bar Chart for Nominal and Ordinal data. The proportion of the Pie represents the category’s percentage in the population or sample. Must identify slices.

What we have learned! Fundamental Features of Graphs Types of graph How to Choose appropriate graph Various Graphics in SPSS Pie chart Bar chart Histogram Frequency Polygon Line chart

Any Questions?

For further Questions: emahmud@manarat.ac.bd