Statistics - Field Sampling: Principles and Practices in Environmental Analysis - page 146

random numbers are needed this is the method best suited to obtaining them.

6.2. VARIATION

When dealing with environmental samples it is common to have variation that is much
larger than the chemist or mathematician would like. For some samples a variation of
±1% might be acceptable; however, with environmental samples a variation of ±10%
may represent a very accurate measurement.
For example, a soil core 1.9 cm in diameter was taken and the pH of each of three
subsamples was measured, first with the fresh sample, then the sample after it was dried,
and again after the sample was dried and mixed. In each case subsamples were mixed
with water (1:1 ratio), and the pH determined using a standardized pH meter. Table 6.3
gives the soil pHs

TABLE 6.3 The pH Measurements of Three Replicate Samples from a Soil Core in the
Original, Air-Dry, and Air-Dry Mixed Conditions

Sample Dry-sieved

mixed
AC12a 5.30 5.32 5.23
AC12b 5.03 5.41 5.28
AC12c 5.01 5.27 5.30
Mean or average 5.11 5.33 5.27
Standard deviation 0.16 0.07 0.04
Note: Unpublished data from senior research project of Luke Baker, Chemistry
Department, Wilmington College, Wilmington, Ohio.

Fresh wet
sample

Dry-sieved
sample

found. The pH values in the second column were obtained after the core, which was
sealed in a plastic bag, was crushed by hand and mixed while still in the bag. The third
column is the same samples after air-drying and sieving (but no further mixing). The
fourth column is the same samples after mixing on a mixer overnight.
It will be noted that variation in the first set of measurements is quite large. Even
though the first measurement appears to be in error, including it gives a better estimation
of the mean than excluding it. Drying the sample leads to a decrease in acidity, while
sieving and mixing had little effect on the pH but decreased the variability. The
difference between a pH of 5.11 and a pH of 5.33 is so small that it would cause no
change in a remediation plan. Later in this chapter these pHs will be compared and their
relationship to each other examined [1].

6.3. POPULATION

In sampling groups of people or fields of corn the concept of sampling a population is