ard 
able 
ul. 
ncy 
at 
of 
Table 3 
Reduction of PU's (and SU's) because of replacement. Number of man-days 
involved in office (preparation grid, area measurement, plot (SU) location) 
and field (transport, plot enumeration). 
  
  
  
  
grid PU SU man-days 
ha SU/PU used x 
M S767 5 used office field | total 
225 2 62 25 23 46 9 14 23 
225 10 62 9 8 80 9 22 31 
  
  
  
  
  
  
  
  
  
  
x) For field transport a car was used. The area is well provided with 
roads and access to sample plots is possible to 300 m. One field crew 
consists of three men. 
From the calculated number of man-days (Table 3) it is obvious that the 
smallest grid cell with the lowest acceptabié number of SU's (225 ha grid 
with 2 SU/PU), notwithstanding the highest s% = 25, is preferable to 
other comhinations 
An excellent variable for the summation list would be volume itself. 
A-priori volumes may be obtained by eye estimations with the aid of 
Stereograms or aerial volume tables. Results from tests, however, show that 
estimated volumes obtained either by means of stereograms or by an aerial 
volume table give similar results to those which use area as a variable. 
Apparently estimated volumes in the PU's are inconsistently determined 
and contain a random error, which, as explained above, raises the standard 
deviation. Only the use of four tables for four age-classes 41-60, 61-80, 
81-100, 101-120 years), instead of one aerial volume table for age-class 
41-120 years, lowers the standard deviation per cent. 
The table of the combined age-classes will show plus and minus deviations 
in the volumes as compared to those of the volumes resulting from the four 
age-classes, in this way creating an increase in the standard deviation. 
If a volume table gives nearly correct plot volumes, this will result in a 
very small standard deviation. However, it is then, unnecessary to use a 
two-stage sampling method, since the plot volumes of randomly photo plots 
lead directly to the desired volume. 
It has been mentioned above, that for a comparison between the various 
methods, all PU's were included and means of 35 samples were taken. To 
conclude, an example may demonstrate the sequence of activities in practice. 
For that purpose only only one random sample, using replacement, comprising 
limited number of (10) PU's is taken from a conifer forest area summation 
list (Table 4) and 2 SU's are systematically located per selected PU. A 
calculation of the mean volume per hectare per PU gives the figures in 
Table 5.