Defining the population
The first step in good sample design is to ensure
that the specification of the target population is as
clear and complete as possible to ensure that all
elements within the population are represented.
The target population is sampled using a sampling
frame. Often the units in the population can be
identified by existing information; for example,
pay-rolls, company lists, government registers etc.
A sampling frame could also be geographical; for
example postcodes have become a well-used means
of selecting a sample. Try to obtain the sample
frame in the most automated way possible for ease
of sampling; for example a database spreadsheet
file.
All sampling frames will have some defects,
despite assurances you may receive from the
holder of the data. Usually there are ways to deal
with this, for example amending the list, selecting a
larger sample and eliminating ineligible items,
combining information from varying sources, or
using estimated or proxy data. If you are having
difficulties identifying a suitable sampling frame
come and discuss this with the Statistical
and Technical Team.
Data Protection Act issues
Often a government database or computer file can be
used to identify the population and select a sample.
You will need to ensure that this data is accurate,
reliable, can be accessed, and that you have
permission to draw a sample. The Data Protection
Act requires us to obtain agreement to use data
which also hold individuals details. Many databases
cannot be accessed because of this or other security
reasons. However, it may be possible to extract
selected information which is sufficient for the
purposes of the study; for example using
summarised data so that the individual cannot be
identified. If you are in any doubt as to your position
in this matter please refer to the Policy Unit.
Contracting out
If you use an outside contractor to carry out the
sample they will normally put forward their
proposed sample design. The design will often
depend on whether you can obtain a suitable
sampling frame from which the sample can be
selected. If you cannot provide a database the
contractor may be able to suggest a sampling frame
to use. The contractor may well use a more complex
sampling design than simple random sampling and
it is important to check that what they have done is
reasonable.
The Statistical and Technical Team hold a database of
contractors previously used by the Office, or you
may wish to search for specific contractors who
specialise in certain fields. A useful starting
point for this is the British Market
Research Associations selectline web
page at:
www.bmra.org.uk/selectline
The Team offer their service as a
reference partner when
drafting the tender for the
work, evaluating the bids,
or assessing the quality of
the work.
6
A sampling frame is
a list of all units in
your population.