A Practical Guide to Sampling

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A Practical Guide to

Sampling

Statistical & Technical Team

This guide is brought to you by the Statistical and
Technical Team, who form part of the VFM
Development Team. They are responsible for
advice and guidance on quantative, analytical and
technical issues.
For further information about the matters raised in
this guide, please contact:
Alison Langham on ext. 7171
This guide is the latest in a series on sampling. It has
been produced in response to a large number of
requests received by the Statistical and Technical
Team relating to sampling matters. The guide aims to
consolidate the information required for you to
complete the survey process from design to reporting.
It provides this advice in an informal and practical
way which should also help you understand the work
of your consultants, and ask informed questions of
the audited body.
This guide replaces the previous guidance
“Use of Sampling - VFM Studies” published in 1992.
Other guides related to this matter:
Taking a Survey (1999)
Presenting Data in Reports (1998)
Collecting, Analysis and Presenting Data (1996)

Contents
Why sample?
Sample design

4
5

Defining the population

6

Data Protection Act issues

6

Contracting out

6

Sample size

7

Weighting a sample

9

Sampling methods

11

Methods, their use and limitations

Selecting an appropriate method
Extracting the sample
Interpreting and reporting the results
Interpreting the results
Reporting the results
Glossary of terms
Appendix 1
Relevant formulae for simple random sampling

11

13
14
15
15
17
18
19

Why sample?

Recent examples

VFM reports require reliable forms of evidence from
which to draw robust conclusions. It is usually not
cost effective or practicable to
collect and examine all the
data that might be
available. Instead it is
often necessary to draw
a sample of
information from the
whole population to
enable the detailed

“Sampling
provides a means
of gaining
information about the
population without the
need to examine
the population
in its entirety.”

examination required to take

place. Samples can be drawn for
several reasons: for example to draw inferences across
the entire population; or to draw illustrative examples
of certain types of behavior.

Caveats
Sampling can provide a valid, defensible methodology
but it is important to match the type of sample needed
to the type of analysis required.
The auditor should also take care to check the quality
of the information from which the sample is to be

Excerpt from Highways Agency: Getting best value
from the disposal of property
HC58 Session 1999-00

drawn. If the quality is poor, sampling may not be
justified.

Do we really use them?
Of the 31 reports published by the end of July of the
1999-2000 session, there are 7 examples of using
judgmental sampling for illustrative case studies and
24 examples of sampling to draw inferences across the
population, of which 19 were the basis for surveys.

Can they provide strong
evidence?
In the Health area, four studies made extensive use of
sampling and survey techniques to form the majority
of the evidence which identified the potential for a one
off saving of up to £400 million and possible annual
savings of £150 million.

4

Excerpt from Charitable funds associated with NHS
Bodies
HC516 Session 1999-00

Sample
design
Sample design covers the
method of selection, the
sample structure and plans
for analysing and
interpreting the results.
Sample designs can vary from
simple to complex and depend
on the type of information
required and the way the sample is
selected. The design will impact upon
the size of the sample and the way in which
analysis is carried out. In simple terms the tighter
the required precision and the more complex the
design, the larger the sample size.
The design may make use of the characteristics of
the...
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