Coordination of spatially balanced samples
Section 2. Notation
Let
and
be a population (subject to change over time)
at time 1 and time 2, respectively, or consider that
and
are two overlapping populations. Consider
samples
and
drawn from
and
using the sampling designs
and
respectively. No restriction about the
sampling designs
and
is necessary to introduce the definitions in
this section: they can be fixed or random size sampling designs, with or
without replacement.
Let
We call
the “overall population”. The set of labels of
the units in
is
We define on
the joint sampling design
used to select a couple
The samples
and
are coordinated if
that is the samples are not drawn independently
(see Cotton and Hesse, 1992; Mach,
Reiss and Şchiopu-Kratina, 2006). Let
and
be the first-order inclusion probabilities of
unit
in the first and second sample, respectively.
It follows that
if
and
if
Thus, it is not necessary to identify
explicitly the subpopulation memberships.
Let
be the joint inclusion probability of unit
in both samples
and
If the samples
and
are selected independently,
for all
Let
be the overlap between
and
which represents the number of common units of
the two samples; it is in most of the cases a random variable. The coordination
degree of
and
is measured by the expected overlap
where
By using the Fréchet bounds of the joint
probability
it follows that
In negative coordination one wants to achieve the left
bound in expression (2.1), that is
while in positive coordination the right
bound, that is
Thus, to optimize the sample coordination
process, the goal is to achieve these bounds, prior to coordination type,
positive or negative. Using the terminology of Matei and Tillé (2005) the left side-part in (2.1) is called the
Absolute Lower Bound (ALB) and the right side-part in (2.1) the Absolute Upper
Bound (AUB).
The focus here is on sample coordination using PRNs. The
PRN method was originally introduced by Brewer
et al. (1972) to coordinate Poisson samples. Poisson sampling with
PRNs reaches the Fréchet bounds given in equation (2.1). Yet, it results
in a random sample size and does not provide spatially balanced samples. In
order to achieve spatial balance, the local pivotal method (Grafström et al., 2012) and the
spatially correlated Poisson sampling (Grafström,
2012) are used. Both sampling designs provide a good degree of spatial
balance (see Grafström et al.,
2012, for some empirical results). Moreover, since both are fixed size
sampling designs (probability proportional to
size sampling, see Särndal, Swensson and Wretman, 1992, page 90), the precision of the estimators is in
general improved compared to Poisson sampling.
In what follows, we consider the sampling designs
and
to be without replacement, and the expected
sample sizes of
and
are denoted by
and
respectively.
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