- 1829 -
RESULTS OF TWO-STAGE UNEQUAL
PROBABILITY (PPS) SAMPLING FOR
TIMBER VOLUME USING
AN ORTHOPHOTO MOSAIC
by
*
D.A. Stellingwerf
General
Assume a two-stage sampling design, in which the primary units (PU's) are
of equal size and the number of secondary units (SU's) per PU are equal.
Randomly selecting the PU's and the SU's in the sample with equal
probability, the variance of the mean volume can be expressed by:
B X r2
"1. GO; - y)
m.n, (m - 1)
Re (-2
2 vy =v
= m (m - 1)
in which y, 7 estimate of maán volume of PU,
y = estimate of overall mean volume,
m = number of PU's in sample,
n, = number of SU's per PU, in sample.
J
It is also possible to consider the individual PU and SU components of the
variance:
Z. 4
e = xm!
J
multiplying with m gives the standard deviation squared:
2
Vy = Vy = ef + — code doh Bl Bak). xD
J
or when using the reduction factors:
s |: +
* Ir D.A. Stellingwerf, Professor Forest Survey, Chairman Division of
Natural Resources, Head Department of Forestry, International Institute
for Aerial Survey and Earth Sciences, Enschede, The Netherlands.