Scmidt-Bleek, F
Tenn. Center for Business and Economic Research University of Tennessee, 1972
Buku Teks
Univ. of Tennessee
The health and safety dept., 1973
Buku Teks
Williams, Bill
John Wiley & Sons, 1978
Buku Teks
Williams, Kate
MacMillan Press , 1989
Buku Teks
Williams, Frederick
Harcourt Brace Jovanovich College Publishers, 1992
Buku Teks