3.6 Why unit-based measurement

Table 3.1: Evaluation of four measurement approaches on seven criteria.
Criterion	Age	Probability	Score	Unit
Independent of age norm	No	No	Yes	Yes
Supports multiple milestones	No	No	Yes	Yes
Latent variable	No	No	Yes	Yes
Robust to milestone skipping	Yes	Yes	No	Yes
Comparable scores	Yes	Yes	No	Yes
Probability model	No	Yes	No	Yes
Defines measurement unit	No	No	No	Yes

This section distinguished four approaches to measure child development: age-based, probability-based, score-based and unit-based measurement. Table 3.1 summarizes how the approaches evaluate on nine criteria.

Age-based measurement expresses development in age equivalents, whose precise definition depends on the reference population. Age-based measurement does not support multiple milestones and does not use the concept of a latent variable.

Probability-based measurement expresses development as age percentiles for a reference population. It is useful for individual milestones but does not support multiple items or a latent variable interpretation.

Score-based measurement quantifies development by summing the number of passes. Different instruments make different selections of milestones, so the scores taken are unique to the tool. Thus comparing the measurement obtained by different devices is difficult. Skipping or adding items require corrections.

Unit-based measurement defines a unit by a theoretical model. When the data fit the model, we are able to construct instruments that produce values in a standard metric.