21
Groeneveld
| PWS-KENT |
1988
23
Walpole, Ronald E.
| Macmillan |
1978
25
26
Bohrnstedt, George W.
| F.E. Peacock Publishers |
1988
27
Chernoff, Herman.
| Wiley |
1959
28
Beals, Ralph E.
| Rand McNally |
1972
29
Woods, Anthony.
| Cambridge University Press |
1986
30
McNichols, Charles W.
| Reston Pub. Co. |
1984