Item Cumulative Probability Curves

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Plots of cumulative probability curves for item categories can be obtained by selecting this option. ConstructMap produces probability curves for P(x=1 or higher), P(x=2 or higher)... through P(x=max score) on the item selected by the user.

  1. Select Item Cumulative Probability Curves from the menu.
  2. Set the Cumulative Probability Map Options just as you set the options for the ICC map.
    Figure 1. Cumulative probability curves for item 1 from Example 2.
    The point at which each curve intersects the y=.5 line is the Thurstonian threshold for the item; that is, it identifies the ability level required to have a 50% probability of achieving the specified score or higher on the item.
  3. The graphic is automatically saved as a jpeg file in the folder specified in step 2, but you can use File – Save As PNG to save it in the Portable Network Graphics format, which may produce a higher quality image for print or web pages and usually produces a smaller file.
  4. Print the graphic using File – Print.
  5. Close the window by clicking on the OK button.