## 5.4 Age profiles of similar milestones

Figure 5.3 displays the percentage of children that pass milestones at various ages. Subject matter experts clustered similar items stemming from different instruments into equate groups. There are 184 equate groups that contain two or more milestones.

Most age profiles show a rising pattern, as expected, though some (e.g. FM17 or EXP11) have one item showing a negative relation with age. Equate EXP26 combines two-word sentences items from seven instruments into one plot. The item difficulties expressed as age-equivalents (c.f. Section 3.1.2, Chapter I) for these cohorts vary between 20-25 months. By comparison, equate group EXP18 (says two words) shows more heterogeneity across cohorts, and is therefore, less likely to be useful for equating. Equate group FM31 (stack two blocks) is another example of a promising example. By comparison, FM38 (stack 6-8 blocks) shows additional heterogeneity. As a last example, consider GM42 (walks alone), which has a similar age profile across cohorts, whereas GM44 (throws ball) or GM49 (walk down stairs) are more heterogeneous.

We could follow different strategies in selecting which equate groups to activate. One strategy would be to include as many equate groups as possible (e.g. all 184 equates) so as to build as many bridges as possible between different instruments. A more selective strategy would be to activate a subset of promising equates and leave others inactive. The following section compares four different approaches.