Placement of the cross-over point has led researchers to distinguish between ordinal and disordinal interactions. In brief, an ordinal interaction has the cross-over of predicted values at the boundary (e.g., Figure 1A) or outside the range of observed values on X1 in the study (e.g., Figure 2A), whereas a disordinal interaction contains a cross-over of predicted values within the observed range of values on X1 as in Figures 1B and 2B. Therefore, the location of the cross-over point is central to differentiating the two forms of linear interaction.
