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Chapter 7 Obtaining and Preparing Samples for Analysis 185

ral distribution of pollutants is anticipated. For example, a plume of waste
leaching from a landfill can reasonably be expected to move in the same di-
rection as the flow of groundwater. The systematic–judgmental sampling
plan shown in Figure 7.4 includes a rectangular grid for systematic sampling
and linear transects extending the sampling along the plume’s suspected
major and minor axes. 5

Stratified Sampling Another combination of the three primary
approaches to sampling is judgmental–random, or stratified sampling.
Many target populations are conveniently subdivided into distinct units,
or strata. For example, in determining the concentration of particulate
Pb in urban air, the target population can be subdivided by particle size.
In this case samples can be collected in two ways. In a random sampling, (a)
differences in the strata are ignored, and individual samples are collected
at random from the entire target population. In a stratified sampling
the target population is divided into strata, and random samples are
collected from within each stratum. Strata are analyzed separately, and
their respective means are pooled to give an overall mean for the target
population.
The advantage of stratified sampling is that the composition of each stra-
tum is often more homogeneous than that of the entire target population.
When true, the sampling variance for each stratum is less than that when the
target population is treated as a single unit. As a result, the overall sampling
variance for stratified sampling is always at least as good as, and often better
than, that obtained by simple random sampling.

(b)
Convenience Sampling One additional method of sampling deserves
brief mention. In convenience sampling, sample sites are selected using Figure 7.3
criteria other than minimizing sampling error and sampling variance. In Effect of sampling frequency when monitoring
a survey of groundwater quality, for example, samples can be collected periodic signals. In (a) the sampling frequency is
1.2 samples per period. The dashed line shows the
by drilling wells at randomly selected sites, or by making use of existing apparent signal, while the solid line shows the
wells. The latter method is usually the preferred choice. In this case, cost, true signal. In (b) a sampling frequency of five
expedience, and accessibility are the primary factors used in selecting sam- samples per period is sufficient to give an accurate
estimation of the true signal.
pling sites.

stratified sampling
A sampling plan that divides the
7 B.2 What Type of Sample to Collect population into distinct strata from
which random samples are collected.
After determining where to collect samples, the next step in designing a sampling
plan is to decide what type of sample to collect. Three methods are commonly used
convenience sampling
to obtain samples: grab sampling, composite sampling, and in situ sampling. The
A sampling plan in which samples are
most common type of sample is a grab sample, in which a portion of the target collected because they are easily obtained.
population is removed at a given time and location in space. A grab sample, there-
fore, provides a “snapshot” of the target population. Grab sampling is easily grab sample
adapted to any of the sampling schemes discussed in the previous section. If the tar- A single sample removed from the target
get population is fairly uniform in time and space, a set of grab samples collected at population.
random can be used to establish its properties. A systematic sampling using grab
samples can be used to characterize a target population whose composition varies
over time or space.