7.2 Defining developmentally on track

In 2006, the World Health Organisation (WHO) published the WHO Child Growth Standards. These standards specify “how children should grow” and form the basis for widely used anthropometric indicators such as stunting and wasting. We advocate a similar approach for child development. More in particular, the following steps:

  1. Measure child development on an interval scale;
  2. Estimate the age-conditional reference distribution for normal child development;
  3. Define the indicator developmentally on track as the proportion above a chosen cut-off.

Step 1 is solved by the D-score. Step 2 borrows from well-tested statistical methodology for constructing growth standards (Borghi et al. 2006). Step 3 can be done in different ways, but a applying a simple cut-off fits easily with regular practice in reporting international comparisons.

Figure 7.1: Illustration of the method to define on-track development (https://d-score.org/dbook-apps/gcdgreferences/).

Figure 7.1 demonstrates steps 2 and 3 in more detail. Click ‘Next’ to advance a series of six steps:

  1. Plot the D-score by age;
  2. Model the relation between age and D-score by an LMS model. In practice, this amounts to smoothing three curves representing the median, coefficient of variation and the skewness.
  3. Present the centile lines for the model;
  4. Plot the age-standardized scores for development (DAZ);
  5. Draw standard deviation lines to indicate the location at \(\pm\) 1 and \(\pm\) 2 standard deviation from the mean;
  6. Count observations above the -2 SD line as on-track. Count observation below the -2 SD lines as off-track (red dots).

Note: These SD lines build upon on a convenience sample. The GCDG cohorts are not representative samples, and the countries are not representative of the world. While we should not over-interpret these references, they play a central role in a stepwise, principled approach to define “developmentally on track.”