Graphical Wright Map
Selecting the Graphical Wright Map produces a graphical representation of Thurstonian threshold values for all items. This map is particularly useful for comparing the relative locations of each step, as shown in Figure 1.
This example uses the Unidimensional Partial Credit sample project.
In this map a histogram of respondent proficiency estimates appears on the left side with the x-axis showing respondent counts. The right side plots item (Thurstonian) thresholds and the x-axis is divided into items, for example “i1,” “i2,” etc. The y-axis for both maps is the logit scale.
Each step level is represented by a different color on this map. In Figure 51 all of the step 1 thresholds are plotted in red, all of the step 2 thresholds are plotted in blue, and all of the step 3 thresholds are plotted in green. We see that items “i1,” “i4,” and “i6” on this map have only two steps, and therefore three categories, while the other items have three steps and four categories.
All of the step 1 thresholds fall within the logit range of -1 to -2, while the step 2 and 3 thresholds are not as consistent. In addition, item “i8” shows little differentiation for steps 1, 2 and 3. In some cases, the Graphical Wright Map can be helpful in establishing cut-points between score levels; in other cases it may show that particular scores require different amounts of ability on each item.
When the user requests a Graphical Wright Map, a display options dialog window is presented. This window is shown in Figure 2, and the options are described in the accompanying Table 1. The settings shown in Figure 2 produced the map shown in Figure 1.
|Figure 2. Graphical Wright Map display options dialog windows.|
|Title||The title you provide will appear at the top of the report. By default, this value is "Wright Map."|
|Display Raw Scores||Select Yes if you want to see Raw Scores on the Wright Map, otherwise select No.|
|Display Item Names||Select Yes if you want to see item names in the report; select No if you want to see item numbers.|
|Show Estimate Type||Select Yes if you want to see the estimate type (e.g., EAP, MLE, etc.); select No if you do not want the estimate type shown in the report heading.|
|Max. Defined Range||Set the maximum logit value to display. This option allows you to reduce or expand the visible area of the Wright Map to be different from the logit range defined in the Estimation Options (it does not change the way proficiency estimates are computed).|
|Min. Defined Range||Set the minimum logit value to display.|
|Bin width relative to defined range||Users can select narrower or wider bin widths for aggregating proficiency estimates.|
|Number of items per page||Enter the number of items to be displayed on each page. This value may require adjustment to display item names or numbers completely.|
|Display item names vertically||Select Yes to display item names vertically to save horizontal space on the map; select No to display item names horizontally.|
|Background Color||Use the adjacent Change Color button to change the background color for the map.|
|Cut-Point Color||Use the adjacent Change Color button to change the color of the alternating horizontal Criterion Zones for the map.|
|Histogram Bar Color||Use the adjacent Change Color button to change the color of the histogram bars.|