- 227 - 
The total cost, however, now equals only: 
t 
(295 + 60) 0.1 = 35. 5 US $. 
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It is very interesting to calculate also the number of field plots involved - without 
3 
using photo plots - for a standard error of S = 0. 5920 m /0. 05 ha. 
0 S 041 2 
From the 120 field plot data as given by STELLINGWERF (1973-1), ' x 12 0 
0. 592 0 2 
= 221 field plots are now calculated to be necessary in order to achieve this 
standard error. The cost for the enumeration of these 221 field plots equals: 
C = 221 x 10 = 2210 US $. 
In fig. 2 two standard error paths are indicated for: 
1) pure field plots without photographs: S- (n ) , 
and for 
2) field and photo plots: S- (n + n ) 
In the figure also two cost paths are given: 
C- (n^) referring to pure field photos, and 
C- (n + n ) referring to field and photo plots. 
^ o P 
The data used for the construction of the line refer to the above used example: so 
3 
for S- (n ) the standard error of the mean plot volume (v = 21. 143 m /0. 05 ha) : 
v g' 
S- = 0. 8041 is taken and for S- (n + n ) the standard error of the mean plot 
v v g p 
120 s p 
volume as calculated from n =175 and n = 120 is used: 
g P 
g 2 = 6 - 2 . 7 _ + o. 2166 2 28 - 14Q3 - 
v n n 
g 
n 
Using the optimum ratio = 9. 72 and the values for n as indicated on the 
"g P 
x-axis, the respective standard error values can be calculated. 
2) Construction of an aerial volume growth regression 
For this application refer to STELLINGWERF (1973-2). For spruce the following 
regression equation from data of n =170 plots was calculated: 
G t = i t + b <cd t - Bd t ) . 
= 2. 2217 - 0. 3131 (cd - 6. 0211) . 
The correlation coefficient r = 0. 7465. 
The standard error of estimate: 
s j = 
g. cd 
0. 3926 m.