@article {di2009multilevel,
	title = {Multilevel functional principal component analysis},
	journal = {Annals of Applied Statistics, PMCID: PMC2835171},
	volume = {3},
	number = {1},
	year = {2009},
	pages = {458-488},
	author = {Di, C and Crainiceanu, C and Caffo, B and Punjabi, N}
}
