Analysis of correlated series of repeated measurements: application to challenge data

Data from an ACTH challenge experiment with veal calves, two short time series of cortisol measurements per animal at 6 and 27 weeks of age, are analysed. Interest is focussed on variation in cortisol profiles both within and between animals. Potential effects of an animals diet and housing system on the profiles are addressed as well. Fully parametric and semi-parametric models, combining individual random effects with effects of diet and housing, were fitted using (approximate) restricted maximum likelihood (employing Laplacian integration). Eigenfunctions were utilized to describe the variation between profiles and the connection between profiles of the same individual. All calculations were performed with standard software. Results of the analysis provides empirical support for the existence of stable individual characteristics mediating reactivity of the adrenal cortex.