Specifications

Appendix B. How Many Soil Water Content Measurements Are Enough
N is the number of water content measurements and L is the acceptable range
as defined by the user. For example, the acceptable water content range might
be r2.5% so L would be 5.0.
Equation 4 relies on a good estimate of the population standard deviation. A
useful approach using these statistical methods is to establish an estimate of the
standard deviation by taking several water content measurements, then use that
standard deviation estimate and the associated t value in [4]. The range is
specified by the user and the calculated N value defines the number of
measurements. The same s and t values are used, but the number of
measurements can be determined for several different ranges.
N
ts
L
 4 4 2 201 174
50
235
22
2
22
2
(. ) (. )
.
.
The calculation of N using the sample standard deviation shows that 3
measurements are required to give a 90% probability that the sample mean
gives the desired r2.5%.
B-3