*2.5. Procedures for Finalization of the IYCD*

In order to finalize the tool, firstly, each core team member reviewed the evidence of the items' performance based on 4 criteria: (i) universality (whether the age at which 50% of children passed each item was similar across countries in terms of location) and discrimination (whether the plotted curve trajectories (slopes) adequately demonstrated increasing attainment by age), as described in Lancaster et al. [13], (ii) inter-rater and test-retest reliability, and (iii) responses from the cognitive interviews (verbatim). Secondly, a joint full team meeting was conducted with the core and country teams. This meeting, which included M.G. (neurodevelopmental pediatrician), T.K. (developmental psychologist), M.J (developmental biologist), G.L. (biostatistician), V.C. (pediatric neurologist), T.D. (pediatric neurologist), M.R. (psychologist), C.R.L.A. (pediatrician) and L.S. (psychologist), was convened to reach consensus as to which items were to be included in the final version of the IYCD.
