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The design of a sampling strategy for a qualitative study is as important as that for quantitative
inquiry. A well-defined sampling strategy that utilises an unbiased and robust frame can provide
unbiased and robust results.
There is a tendency, particularly within a quantitative environment, to consider that the sampling
strategy for qualitative research is of lesser importance to that where statistical inference is
required. Indeed, it is not unknown for those unfamiliar with qualitative research methods to
suppose that no more than a convenience sampling strategy is applied. That is to say the
researcher makes no attempt, or only a limited attempt, to ensure that the sample is an accurate
reflection of the population.
This paper attempts to show why it is as important to develop a robust sampling strategy, from a
well-constructed sampling frame, for qualitative research practice. The paper also discusses how
the Office for National Statistics (ONS) puts theory into practice using a qualitative ‘Respondent
Register’ developed for use for sample frame construction for qualitative social research.
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It is difficult to discuss the design of qualitative sampling strategies without first discussing the
nature and purpose of qualitative research, and how its approach differs from quantitative
research.
The terms ‘quantitative’ and ‘qualitative’ research are used as overarching categories covering a
wide range of approaches and methods within each. However, the very bases of those approaches
differ.
Quantitative research, by definition, implies a measurement or numerical approach. The
methodology employed is based on the testing of hypotheses deduced from theory. Using
statistical inference the results may be generalised to the population.
Qualitative research aims to provide an in-depth understanding of the world as seen through the
eyes of the people being studied. It aims not to impose preordained concepts; hypotheses and
theory are generated during the course of conducting the research as the meaning emerges from
the data. Statistical inference is not the objective, although within government, results are used to
inform policy and therefore some form of generalisation or transferability is implicit.
Qualitative research may stand alone or in conjunction with quantitative research, used before,
along side or after. Where the studies are associated then the sampling strategy for both should be
considered at the same time.
220
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Different qualitative sampling strategies may be used at different stages of the research, or for
different research purposes.
Questions which the researcher should ask themselves at the outset, and which will inform the
design of the sampling strategy, are the similar for both quantitative and qualitative research.
They are:
• What are the research objectives?
• What is the target population?
• Who should be excluded from the sample?
• Who should be included in the sample?
• What is the budget?
• What is the reporting time period?
• How many qualified researchers are available to work on the project?
• What sampling technique(s) should be employed?
• How are the data to be analysed?
• What data collection methods should be employed?
• What are the sample criteria?
• How long will the interview be?
• What size should the sample be?
• What should be used as the sampling frame?
• How should potential respondents/participants recruited?
All of the above are interdependent, however some of the questions require a more detailed
discussion with regard to their application in a qualitative research environment.
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Time, budgetary and other resource constraints may impact on the qualitative sample design
but should not be allowed to undermine it. The nature of the data collection method (e.g.
cognitive, in-depth, or group interview), the human resources available to the project and their
skills base are also important considerations.
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The sampling technique employed is a crucial element of the overall sampling strategy.
At this point it is important to understand why probability sampling is inappropriate for
qualitative research. In probability sampling members of the research population are chosen at
random and have a known probability of selection. Groups are represented in the sample in their
true proportions; or, where unequal probabilities are used the data are reweighted back to the true
proportions. The aim is to produce a statistically representative sample, suitable for hypothesis
testing.
Qualitative research uses non-probability sampling as it does not aim to produce a statistically
representative sample or draw statistical inference. Indeed, a phenomenon need only appear once
in the sample.
221
Purposive sampling is one technique often employed in qualitative investigation. With a
purposive non-random sample the number of people interviewed is less important than the criteria
used to select them. The characteristics of individuals are used as the basis of selection, most
often chosen to reflect the diversity and breadth of the sample population.
However, there are different approaches to purposive sampling some of which focus on different
aspects of the sample members, cases are chosen because they are considered more extreme, for
example. One form of purposive sampling is ‘theoretical sampling’, developed from the
‘grounded theory’ approach (Glaser and Strauss, 1967). The term ‘grounded theory’ expresses the
idea that theory is generated, through an iterative process, involving the continual sampling,
collection and analysis of data to inform the next stage of the sample design, until ‘theoretical
saturation’ is achieved; that is, no new ideas or theories emerge. The iterative nature of the
theoretical sample design is important. It gives the researcher the opportunity to analyse the data
as the sampling progresses and means that the researcher can add to or change the emphasis of the
sample design, and in doing so ensure robustness of the theories generated. It is therefore valuable
to have considered the analysis technique early on in relation to the qualitative sampling strategy.
Sometimes theoretical hypothesis generation is not the primary aim of the research. Where the
sample population is clearly defined, such as when testing already operational survey questions,
and where resource and time constraints are in place, then a more constrained purposive sampling
strategy can be devised that avoids iteration and does not necessarily achieve saturation, on the
grounds of diminishing returns.
Whatever approach is used, some advance knowledge of the population under investigation is
necessary when carrying out purposive sampling.
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A decision will be required as to the sample selection criteria. That is, what characteristics will
need to be reflected in the sample population to address the research question. The decision on
which criterion to use will be informed by the policy advisor and other subject specialists, as well
as a review of the current literature. The researcher will need to know whether particular sub-
groups need to be included to ensure breadth. The criteria used may be based on demographic
characteristics or behaviours or attitudes, and will need to be prioritised if purposive sampling is
to be employed. This is partly influenced by the fact that qualitative research is often, but not
always, based on a relatively small number of cases so it may not be possible to include all of the
sample criteria in the sample design. Some criteria may be considered more important than others
in relation to the research objectives.
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The intensity and therefore the length of the qualitative interview will also impact on the design
of the qualitative sampling strategy and the decision of sample size. Longer interviews may
provide more data than shorter interviews. A decision may be taken, depending on the nature of
the study, to conduct a larger number of shorter interviews or a smaller number of longer
interviews.
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A feature of qualitative sampling is this fact that the number of cases sampled is often small. This
is because, as mentioned earlier, a phenomenon only need appear once to be of value. There is no
need for scale as there is no need for estimates of statistical significance. Furthermore, because
222
qualitative investigation aims for depth as well as breadth, the analysis of large numbers of in-
depth interviews would simply be unmanageable because of a researcher’s ability to effectively
analyse large quantities of qualitative data. However, the small-scale approach only works if the
researcher has a strong sampling strategy (Ritchie and Lewis 2003).
The issues that should be considered when determining the sample size for qualitative
investigation are dependent on the heterogeneous or homogeneous nature of the sample
population, or requirements of the data collection methods employed; for example, focus groups
tend to be more productive and manageable if participants have some commonality.
The number of selection criterion required and the degree to which criteria are nested (dependent
on whether certain characteristics are to be controlled for e.g. age), are important considerations.
The intensive nature of the study; whether multiple samples are required, the inclusion of a
control sample for instance; and the resources available to conduct the study, are also important
for determining sample size.
To provide some idea of the scale of qualitative investigation one might expect to achieve
between 20 and 50 interviews for a one-to-one investigation and around 60 to 100 participants at
group interview, depending on the research question.
The size of the sample required will of course also feed into the decision about type of sampling
frame to use.
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A sampling frame is a list or map that identifies most units within the target population. (Missing
units are referred to as undercoverage.)
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When evaluating the effectiveness and efficiency of any sampling frame for qualitative research,
it is important, as with quantitative research, to consider whether the frame is comprehensive.
That is, all of the target population are included. The full range of dimensions, and information
needed to inform the sample selection, should be covered. This is because sections of society
missing from the frame may have different characteristics and indeed different behaviours,
opinions and attitudes from those covered by it. This undercoverage may affect the results if
associated with the subject of enquiry for example, phenomena may not be raised or survey
questions not tested thoroughly.
It is however, also important to consider overcoverage. Sample members may be listed more than
once, or the list may contain members considered out of scope for the purposes of the study. Their
inclusion in the study could impact on the findings and indeed on resources and the ultimate cost
of the project.
Furthermore, the frame should also contain sufficient numbers in each sub-group to provide the
sample size required, as not everyone who is eligible will be willing to take part. Three or four
people may be contacted who fulfil the sample criteria before one agrees to take part.
A practical consideration is whether the frame can be easily manipulated in order to identify those
with the relevant characteristics.
223
As with frames for quantitative research, geographical clustering is important because if the
population are highly dispersed then fieldwork will be more resource intensive. It is also
important to know whether the potential respondent contact details are complete and up-to-date.
Lastly, whether the time and cost involved in using the frame is justified.
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There are basically two types of frame (or list) available for social research practice. These are
existing lists that can be used as frames, perhaps after some manipulation, or frames that need to
be constructed.
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Existing frames usually comprise records which were constructed for administrative purposes, for
example, a published list of GP surgeries. As such, they tend not to have been designed with
research purposes in mind and may not be very well maintained from a research perspective.
Furthermore, some administrative records, for example, benefit records, will be covered by data
protection and confidentiality issues, which can make them difficult to access.
Existing survey samples can provide a frame, but they may also not have been designed with the
current research interest in mind. (Although if the qualitative research is a follow-up to a
quantitative survey and has been planned as such, then the qualitative sample criteria can be built
into the quantitative survey in advance.) Existing survey samples may be affected by
undercovereage and response bias, not least because they are dependent on the survey response rate.
For ethical reasons, the survey respondent will have been required to give permission for a survey
organisation to contact them again for future research. Not everyone taking part in the original
survey will have given their consent, particularly if the first survey interview was long or not
interesting. There are therefore additional attrition issues to be considered. Ethical considerations
might also include the burden on the respondent taking part in more than one interview.
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Where an existing frame or list is not available then the researcher may have to create their own.
Sampling may then take place directly from the constructed frame or indirect methods of
obtaining a sample may be used to either construct or to supplement the frame. In qualitative, as
with some quantitative, studies the actual sampling and recruitment may take place in the field, as
potential respondents fulfil the sample criteria and the overall sampling strategy.
The use of household or organisation screening methods is not uncommon. However, both are
subject to the ‘gatekeeper syndrome’ where the person initially contacted may not have available,
or may be unwilling to provide, information about others. Particularly where an organisation is
concerned, there may be a tendency for the organisation’s representative to select potential
respondents who are in more regular contact with the organisation or who may present the
organisation in a good light.
Another method of frame construction that can be useful, particularly where the population is
clustered geographically, is focussed enumeration. Here interviewers call at a certain number of
addresses living either side of respondents who fulfil specified criteria. They will administer a
screening questionnaire and attempt to gain an interview with people who may also fulfil the
specified criteria. This method is more effective where certain characteristics are known to cluster
such as high income earners or certain ethnic groups.
224
Snowballing, where respondents tell the researcher about others they know with the same or
specified characteristics, is a method also used in qualitative research. As potential respondents
often know each other, the disadvantage with snowballing is that a sampling frame created using
this method alone may be prone to bias. This is because those who know each other may have
similar behaviours and attitudes or may influence each other in relation to the research. Those that
are missed may have quite different characteristics. As a result the sample may not be particularly
diverse and important phenomena may be missed. It is therefore usually used as a supplement to
other methods.
Advertising for potential respondents is not advised unless other methods of frame construction
are unavailable. If used, a rigorous screening process should be in place but even then the sample
members are, to a certain extent, somewhat self-selecting.
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The method of respondent recruitment and its effectiveness is also an important part of the
sampling strategy.
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For each recruitment project, a comprehensive set of specifications should be drawn up,
particularly if an external recruitment agency is to be used.
It is therefore of paramount importance to provide a detailed specification of the project to those
doing the recruiting. This detailed specification is also useful for the research commissioners in
order to alleviate fears about the quality of the research.
The recruitment specification will detail the recruitment strategy and explain the purpose of the
research and how the research will be conducted and for whom. It will also provide a schedule of
events, as well as the detail of any screening questionnaire. Following this, a quota is drawn up
according to the sampling strategy and, as people are contacted and meet the criteria and agree to
take apart, a note is made of which quota they fill. This in turn informs the recruitment of the next
person on the list.
At this point it is worth briefly discussing the use of recruitment agencies. Many research
organisations use professional recruitment agencies to conduct respondent recruitment for
qualitative research. This approach can certainly save time and be cost effective since such
agencies specialise in recruitment and have the in-house expertise to manage it. However, it is
important periodically to take the time to quality assure the work of these agencies, either directly
or though discussions with respondents about their experience of the recruitment process.
It is also worth discussing the use of incentive payments in relation to recruitment for qualitative
research. Due to the intensive (at least cognitively) and sometimes intrusive nature of the interviews,
incentive payments are commonplace in qualitative research. At the ONS this is kept to a minimum
and is really no more than a token of our appreciation for the time people have spent talking with us,
and to cover any travel costs involved on the part of the interviewee. It is important not to mention
the incentive to potential respondents at the outset as there is a chance that someone may sift
themselves into the study on that basis alone. Certainly in the United States for example, attendance
at focus groups is becoming a regular way of earning money. Such professional focus group
attendees are to be avoided as their participation will most likely bias the results.
225
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It is important to document the outcome of the recruitment process in the same way as it is
important to document outcomes in quantitative survey research. This is because it is essential to
know whether there is any attrition or deficiencies and biases in the sample and the sampling
frame used with respect to the research objectives. Furthermore, the information is valuable for
informing future research using the same frame.
It is important to know the characteristics of those who refuse to take part, particularly if their
refusal is related to the research topic. Furthermore, their refusal may, in part, be a result of the
recruitment strategy, which may need revision. Information about non-contacts is important to
inform the recruitment strategy, or to check the accuracy of contact details on the frame.
Information about those contacted but who were ineligible for the purposes of the study is useful
for checking the effectiveness of the sampling frame. Their agreement to be contacted again in the
future could also be sought, providing further valuable information.
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At the ONS, respondent recruitment is conducted by the researchers themselves and not by an
outside agency. A major part of the work carried out by the ONS qualitative research unit relates
to testing social survey question(naire)s, including the Census, administered to the general
population. We have been investigating ways of constructing sampling frames for qualitative
social studies.
The ONS continuous social surveys can provide details of survey respondents but, particularly
where the survey is run annually, the information is often difficult to retrieve quickly and on an ad
hoc basis. Furthermore, providing a list of potential respondents is not a survey manager’s
priority.
We have therefore looked to the National Statistics Omnibus survey to provide a list of potential
respondents and their details. The Omnibus is a multi-purpose survey, run on a monthly basis,
covers the whole of Great Britain and selects one person living in a private household, aged 16 or
over, for interview. Data are available within a few weeks of fieldwork completion.
The Omnibus Survey uses the Royal Mail’s small user Postcode Address File as its sampling
frame, which aims to include all private household addresses in Great Britain. A new sample of
100 postal sectors is selected at random each month. The postal sectors are selected with
probability proportionate to size and, within each sector, 30 addresses are selected randomly.
Around 1800 individuals respond to the survey each month
∗
.
A potential respondent register is being built from details of those who responded to the Omnibus
survey and gave their contact details and permission to be contacted again in the future by the
ONS. Every month around 1470 (82%) individual contact details are added to the register.
Sampling frames for specific projects can then be constructed from the register.
∗
It should be noted that since April 2005 the number of sampled postcode sectors has been reduced to 67. From
this date onwards just over 1200 interviews are achieved each month.
226
The register is currently stored as an SPSS database. Details of potential respondents remain on
the database for a period of four years, after which time they are deleted. This is because around
1/3 of respondents from the original monthly Omnibus sample, are likely to have moved over that
time period (a higher proportion among the private renter population), and details may then be out
of date. Furthermore, when respondents have taken part in a maximum of two qualitative studies
they are not contacted again; so respondent burden is minimised. Contact is usually made by
telephone followed by a face-to-face interview.
As discussed earlier, it is important for the researcher to be aware of any potential deficiencies in
the register or frames produced from it, particularly if those deficiencies relate to the research
question or subject under investigation. The advantages and disadvantages of the register in this
respect are detailed in section 3.2.
The ONS respondent register contains the following information about those who agreed to be
contacted:
• Around fifty classificatory variables including both household and individual level
information such as household type and composition; tenure; number of cars; respondent
age and sex; marital, education, health and employment status; socio-economic
classification; ethnic group and income band.
• Geographical information such as postcode and postcode sector, Local Authority and
Government Office Region.
• Comments from Omnibus interviewers about the household such as how to find the
address.
• A note of the Omnibus topic areas covered each month.
• Address details (100% coverage of Omnibus sample agreeing to recall).
• Respondent name (100% coverage).
• Telephone number (96% coverage).
• Email address (19% coverage).
• Number of times respondent contacted via the register.
• Dates when contacted.
• Times of contact.
• Contact outcomes.
• Number of times respondent participated.
• Details of project participated in.
• Dates of participation.
• Name of researcher conducting interview.
The register is kept and maintained in a secure environment, access procedures have been verified
and agreed with those responsible for data protection at ONS. Respondent confidentiality is
paramount. Only two members of staff have access to the database.
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We have recently begun to investigate an enhancement to the register that is intended to make it
more efficient as a frame for qualitative purposes. The use of geographical mapping software
means that each potential respondent, with the required sample characteristics, can be mapped at
postcode level and a located across the country. This means that the spread of potential
respondents with the required characteristics can be visualised. Areas where potential respondents
227
cluster are then visible and can be easily targeted for recruitment. This is especially useful when
recruiting for face-to-face focus groups since the groups need to come together in one convenient
location. It is more difficult to encourage a dispersed group to meet due to the time and cost
involved in getting together. In addition, outliers, that is potential respondents who do not cluster,
are also important in assuring breadth of coverage. These people may be different from those who
cluster and should also be included in the research, at least in the initial stages
∗
.
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The major benefit of having a qualitative potential respondent register such as this is the
considerable efficiency gains for the office both in terms of quality and resource. However, the
decision to use the register will depend upon the research objectives and requires an
understanding of the strengths and weaknesses of the register, which are detailed below.
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• Potential respondent contact details are up-to-date. Telephone numbers are available in
96% of cases which makes the initial respondent contact simple, timely and less costly
than contact made by post or in person.
• A large number of demographic and classificatory variables means that selection criteria
can be varied and relevant.
• The size of the register (around 47,000 cases) means that sub-sampling for frame
construction is both possible and effective.
• There is diversity and breadth of coverage since the original Omnibus sample design was
based on random probability.
• When implemented, the mapping software will be useful in identifying where people who
meet the sample criteria cluster and additional sampling methods, such as focussed
enumeration or snowballing, may achieve a greater success rate.
• Even if the Omnibus respondent is unable to take part in the subsequent qualitative study
there may be others in the household who fit the selection criteria and are willing to be
interviewed.
• The register can be used as a starting point for frame construction and prior to
administering a screening questionnaire.
• The fact that the Omnibus has a random sample design and only those selected can be
included for interview means that potential qualitative respondents are not likely to be so
called ‘professional qualitative respondents’. Indeed, many have never taken part in social
research prior to being interviewed on the Omnibus.
∗
It should also be borne in mind that the original Omnibus sample has a clustered design.
228
• Although the register is depleted through use for various studies it is also refreshed each
month as a new Omnibus sample is added.
• Outcomes of any recruitment process are recorded which inform subsequent studies and
periodic assessments of the completeness of the register can be made by comparing
distributions with other data sources. This means that the researcher is informed at the
outset of any potential deficiencies in the frame.
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• Ensures efficient use of research resource as less time is spent identifying or constructing
a suitable sampling frame.
• Ensures efficient use of research resource in identifying regions or areas to either target or
avoid for recruitment.
• If implemented the additional use of the mapping software will be able to inform the
development of the sampling and recruitment methodology.
• The mapping software would also aid decision making related to cost and timeliness. A
more dispersed sample will require greater resource input when interviews are to be
conducted face-to-face as researchers will have to travel further to interviews.
• The issues that are raised in recruitment, including those related to the geographical
mapping information, may reveal issues that can further feed in to understanding during
the research analysis or project reporting.
• From a practical perspective the SPSS database is easy to access and to manipulate. Sub-
sampling frames can be produced quickly, particularly bearing in mind the number of
classificatory variables included on the file. The ease of manipulation also means that it is
simple to quality assure the sampling and recruitment process
∗
.
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• The fact that respondents have already taken part in an Omnibus interview and have been
visited by an ONS interviewer means that greater confidence can be placed in the personal
safety of the qualitative researchers when visiting people in their own homes.
• Because respondents have already taken part in the Omnibus survey and have agreed to
ONS recall, respondents feel less pressurised to take part and more confident in the
research process.
• Again, because respondents have already been through the Omnibus interview process
they generally require less explanation as to what to expect in relation to the practical
elements of taking part in the research e.g. making appointments.
∗
It is important to conduct probity checking of qualitative recruitment and interviewing in the same way as is
conducted for quantitative inquiry.
229
• The previous experience of the Omnibus interviewer and comments such as how to find
an address are also useful for the qualitative researcher.
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• The register as a sampling frame is potentially biased in the sense that it only lists those
who responded to the original Omnibus survey (65% on average) and then only those who
then agreed to be contacted again in the future (80% on average).
• As the Omnibus is a survey of people aged 16 years or over, living in private households
in Great Britain, the register can only list people in those circumstances and would not
include people living in institutions. This means that the register alone may not be suitable
for a study of the elderly or disabled for example.
• Despite the large number of cases listed on the register it may still not be suitable for
studies of sub-groups of the general population where numbers could be very small,
particular ethnic groups for example.
• Classificatory variables provided by the Omnibus are factual in nature. However, the
sample criterion may be behavioural or attitudinal in nature. Therefore a further screening
questionnaire would be required.
• Although refreshed each month and containing large numbers of potential respondents,
the register may, in the longer term suffer from over use and resulting bias. This is
because respondents are removed from the register after taking part in research on two
occasions. So if a number of studies are carried out using the same selection criteria this
could deplete the frame of certain types of respondent.
(IILFLHQF\ORVVHVIURPDUHVHDUFKSHUVSHFWLYH
• Potential respondents have already taken part in an Omnibus interview and although
agreed to recall, there is still an increase in respondent burden.
• Respondents contacted to take part in a qualitative study are usually offered a payment in
the region of £15 to £30 depending on the nature of the study and data collection method
used. Those who took part in an Omnibus interview were not provided with any incentive
payment, other than a book of first class stamps. This may raise a question in the
respondents mind about why they were not paid to take part in the original survey
interview.
• The ease and accessibility of the register may mean that over time researchers become
reliant on it when thinking about the most appropriate recruitment strategy, choosing to
use the register as an easy option rather than it being the most appropriate option.
230
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An effective sampling strategy is as important for qualitative as it is for quantitative research. The
research objectives, sample population, resources available and reporting period, sampling and
analysis techniques, data collection methodology, sample criteria, interview length sample size,
sampling frame and recruitment method, all feed into the design of a qualitative sampling
strategy.
Fundamental to the sampling strategy is the choice of sampling frame. Obtaining an appropriate
sampling frame from existing lists or constructing a frame can be difficult and time consuming,
although there may be no alternative. The decision is very much dependent on the nature of the
study and the target population.
However, where studies involve members of the general public and sampling criteria are factual
in nature the ONS qualitative respondent register provides a simple yet comprehensive and
effective list containing around 47,000 potential respondents and 50 potential selection criteria,
from which frames can be constructed. Furthermore, screening interviews can be conducted to aid
frame construction, perhaps where relevant research criteria are not included.
The fact that the register contains such a large proportion of potential respondents while in
contrast qualitative studies are small in scale, one might question why register bias should be
considered at all. But misuse, overuse and poor documentation can certainly affect the quality of
the register and sub-sampling of frames, and hence the quality of the sampling strategy employed.
The register is still under development and as such should currently be considered a prototype
tool. However, it has proved effective as a source for sample frame construction for a number of
social research projects already carried out by the ONS qualitative research unit, providing
evidence of its value, encouraging further development and its formal operationalisation as a
necessity.
231
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Thanks are due to the ONS Omnibus research team, in particular Victoria Peacey, Laura Rainford
and Sandra Short, without whose support the register could not have been constructed.
Thanks are also due to Zeeshan Rahman and Colin Hand in Data Collection Methodology Branch
at ONS who helped to build and now manage the register.
Finally, thanks to Allen Gower from Statistics Canada for his input with regard to information
about respondent recruitment specifications.
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