Select by clicking on "Empirical ICC" or from the Graphs menu.
Line: This shows the empirical (data-descriptive) item characteristic curve. Each black "x" represents observations in an interval on the latent variable. The "x" is positioned at the average rating (y-axis) at the average measure (x-axis) for observations close by. "Close by" is set by the empirical slider beneath the plot. The blue lines are merely to aid the eye discern the trend. The curve can be smoothed with the "smoothing" slider. The Points button controls whether points+lines, points or lines are displayed.
Buttons are described in Graph window.
The markers on the empirical ICCs are the data-points. The lines between the markers are interpolations.
Think of the empirical ICC for an item. This summarizes the scored responses to the item. There is one scored response by each person. Each person has an ability. The abilities are stratified into ability ranges. The size of the range is shown by the slider below the middle of the graph. In each ability range, one marker is plotted. The x-axis is the average ability of the persons in that range. If there is no person, then there is no marker. The y-axis is the average of the scored responses by the persons. The low end of the lowest slot is the lowest person ability (- item difficulty) observed for the item.
The lines between the markers are to help our eyes see the pattern of the markers more clearly. If each ability range is very wide, then there may be only one or two markers. If the each ability range is very narrow, there could be one marker for each observation. There is an art to adjusting the width of the ability ranges to produce the most useful empirical ICC.
