4.1 What is an equate group?
An equate group is a set of two or more milestones that measure the same thing in (perhaps slightly) different ways. Table 3.2 contains an example of an equate group, containing items that measure the ability to form two-word sentences. Also, Figures 3.2 and 3.3 show examples of equate groups.
Equate groups vary in quality. We can use high-quality equate groups to link instruments by restricting the difficulty of all milestones in the equate group to be identical. Equate groups thus provide a method for bridging different tools.
Figure 4.1 displays items from three different instruments with overlapping sets of milestones. The shared items make up equate groups, as presented by the arrows between them. In the example, all three instruments share one milestone (“walk alone”). The “sitting” and “clap hand” items appear in two tools. So in total, there are three equate groups.