Planned Missing Data Designs for Spline Growth Models in Salivary Cortisol Research

Salivary cortisol is often used as an index of physiological and psychological stress in exercise science and psychoneuroendocrine research. A primary concern when designing research studies examining cortisol stems from the high cost of analysis. Planned missing data designs involve intentionally omitting a random subset of observations from data collection, reducing both the cost of data collection and participant burden. These designs have the potential to result in more efficient, cost-effective analyses with minimal power loss. Using salivary cortisol data from a previous study (Hogue, Fry, Fry, & Pressman, 2013 Hogue, C. M., Fry, M. D., Fry, A. C. and Pressman, S. D. 2013. The influence of a motivational climate intervention on participants’ salivary cortisol and psychological responses. Journal of Sport & Exercise Psychology, 35: 85–97. [PubMed], [Web of Science ®] , [Google Scholar]), this article examines statistical power and estimated costs of six different planned missing data designs using growth curve modeling. Results indicate that using a planned missing data design would have provided the same results at a lower cost relative to the traditional, complete data analysis of salivary cortisol.