@misc{Pinheiro2017,
author = {Pinheiro, J. and Bates, D. and DebRoy, S. and Sarkar, D. and R Core Team},
title= {nlme: Linear and nonlinear mixed effects models: R package version 3.1–131},
year = {2017},
URL = {http://CRAN.R-project.org/package=nlme},
publisher = {World Health Organization},
address = {Geneva},
}