Cambridge University Press, 2018
Majalah, Jurnal, Buletin
Titmuss, Richard M
Longmans Green and Co, 1950
Buku Teks
Artis, M.J.
Department of Economics, University of Manchester, [date of publication not identified]
Buku Teks
Huttman, Elizabeth Dickerson
McGraw-Hill , 1981
Buku Teks
A Spectrum book, 1961
Buku Teks