APPENDIX

Calculations for Predicted PPVT-R Raw Scores From the Growth-Curve Model

PPVT-R Raw Score at Given AgeEquation From Growth ModelEstimated Score
3 y12.2 + 17.4 (3 − 3) − 0.7 (3 − 3)212.2
4 y12.2 + 17.4 (4 − 3) − 0.7 (4 − 3)228.9a
11 y12.2 + 17.4 (11 − 3) − 0.7 (11 − 3)2106.6
12 y12.2 + 17.4 (12 − 3) − 0.7 (12 − 3)2112.1b
  • On the basis of the computed estimates from the growth-curve model, the following equation could be derived: 12.2 + 17.4 (age − 3) − 0.7 (age − 3)2. Age is centered around 3 years (the age at the beginning of the study period), to facilitate interpretability of the parameter estimates.

  • a PPVT-R raw score increase from 3 to 4 years = 16.7 points.

  • b PPVT-R raw score increase from 11 to 12 years = 5.5 points.