223 
c g 
corr.coeff.( r) 
As it may be assumed that: 
2 
I 
( C V 
c ) 
n 
(n - 1) 
p 
V(c - c )' 
/ n n 
S S_ 
(n„ - 1) 
.2 2 
b. s 
c 
n 
Therefore: 
n 
_S 
n 
P 
L<v 
V ) 2 
2 2 
r . s 
V 
I< C n - 
£_ 
Y (C - 
n 
g 
& 
- ,2 
iZ 
v c g 
il-JLl 
2 
r 
(ng 1 - 1) 
For these derivations reference is made to LOETSCH & HALLER (1954) and 
COCHRAN (1963). In fig. 1 the optimum ratio ng/np can be found for a given 
correlation coefficient (r) and a given cost ratio cp/cg. 
APPLICATION 
1) Construction of an aerial volume regression. 
Referring to the publication of STELLINGWERF (1973-1) in which the application 
of an existing volume regression for spruce is discussed, the following equation 
was used: 
V = 19.9474 + 0.2166 (c [ - 51.6571), 
g 
in which n =175 plots of 0. 05 ha each, 
g 
The correlation coefficient is r = 0. 6845. 
3 
The standard error of estimate s = 6.27 m /0. 05 ha. 
v. c 
Substitution of c. = c (out of n = 175 + 120 = 295 photo plots), gives: 
l g n p P 
v = 19.9475 + 0.2166 (51.2542 - 51.6571) 
295 
= 19. 86 m 3 /0. 05 ha.