4.3 Probability of passing a milestone given D-score

Figure 4.2: Empirical percentage of passing each milestone in the DDI against the D-score (Source: SMOCC data, 2151 children, 9 occasions).

Figure 4.2 is similar to Figure 4.1, but with the horizontal axis replaced by the D-score. The D-score summarizes development into one number. See 5.3 for a detailed explanation on how to calculate the D-score. The vertical axis with per cent pass is unchanged.

The percentage of successes increases with D-score for all milestones. In contrast to Figure 4.1 all curves have a similar slope, a desirable property needed for an interval scale with a constant unit of measurement (c.f. Section 3.4).

How can the relation between per cent pass and age be so different from the relation between per cent pass and the D-score? The next section explains the reason.