Second, the assumption about measurement precision and intervals at all points on a dimension is also of key importance. If this assumption were incorrect, point and interval estimates of the cross-over point could be biased. This concern is difficult to evaluate empirically, but must be borne in mind. Third, drawing firm conclusions about the ordinal or disordinal nature of the interaction presumes that the full population range on X1 is observed in a study or at least considered. If range restriction on a predictor occurs, the range of values observed in a study is narrower than in the population. A cross-over point that falls outside the range of X1 values observed in a study, but still falls within the population range of X1 values, may require special care when characterizing the interaction as ordinal or disordinal.
