Child factor in measurement dependability

A primary consideration in longitudinal growth studies is the identificatio n of growth from error components. While previous research has considered m atters of measurement accuracy and reproducibility in detail, few reports h ave investigated the errors of measurement due to aspects of the physiology and cooperation of the child. The present study directly assesses this sou rce of measurement undependability for the first time. Investigation of tot al measurement error variance in 925 recumbent length replicates taken over stasis intervals in growth identifies that between 60% and 70% of total me asurement unreliability is due to a child factor undependability. Individua l differences are significant and longitudinal growth analyses should consi der two to three times the technical error of measurement statistic as a re asonable estimate of the total unreliability for any single measurement of an infant's recumbent length. These results raise issues regarding analytic methods as applied to serial growth data. (C) 2001 Wiley-Liss, Inc.