9.3 D-score for international comparisons
The D-score is a universal scale of early child development. The D-score does not depend on a particular instrument. Instead, we can calculate a D-score as long as appropriate difficulty estimates are available for the tool at hand. This feature makes the D-score methodology flexible and helpful for international comparisons.
Of course, the ideal situation for international comparisons would be that all countries collect child development data in the same way. In practice, this ideal may be difficult to achieve. Also, we cannot change past data. In these less-than-ideal worlds, the D-score presents a convenient, conscientious and timely alternative.
As an example, we outlined a generic strategy on how to advance on SDG 4.2.1. We use the D-score to operationalise the concept developmentally on track. We calculated age-conditional references of the D-score, analogous to the WHO Multicentre Growth Reference Study. We may then define cut-off values. Children above the cut-off then count as developmentally on track.
While we highlighted the principles, much work still needs to be done. First, there are over 150 instruments for child development, and our current key covers only a fraction of these. We are actively expanding the key using additional data, so as time passes the coverage of tools will go up. Second, we calculated the references on a mix of studies, some of which include special populations. Thus, we cannot interpret the current reference values as portraying normal development. We hope that the inclusion of healthy population data will improve the usefulness of the references as a standard for child development.