@article {heckman_comparing_2000,
	title = {Comparing the shapes of regression functions},
	journal = {Biometrika},
	volume = {87},
	number = {1},
	year = {2000},
	pages = {135{\textendash}144},
	url = {http://biomet.oxfordjournals.org/content/87/1/135.short},
	author = {Heckman, Nancy E. and Zamar, Ruben H.}
}
