Abstract
References
1.
Friedman
G.D.
Primer of epidemiology , Scarborough, Ont. : McGraw Hill , 1974 , p. 47 .
2.
MacMahon
B.
,
Pugh
T.F.
Epidemiology: principles and methods. Boston : Little, Brown and Company , 1970 : 44 .
3.Epidemiology: principles and methods. , pp. 43 –44 .