Kirkwood, R. N., Brandon, S. C., Moreira, B. de S., & Deluzio, K. J. (2013). Searching for Stability as we Age: The PCA-Biplot Approach. International Journal of Statistics in Medical Research, 2(4), 255–262. https://doi.org/10.6000/1929-6029.2013.02.04.2