@article {heckman2000comparing,
	title = {Comparing the shapes of regression functions},
	journal = {Biometrika},
	volume = {87},
	number = {1},
	year = {2000},
	pages = {135{\textendash}144},
	publisher = {Biometrika Trust},
	author = {Heckman, Nancy E and Zamar, Ruben H}
}
