8/2/2019 Qualitative Sampling

1/4

Family Practice Oxford University Press 1996 Vol. 13, No. 6Printed in Great Britain

Sampling for qualitative researchMartin N MarshallMarshal l , MN. Sampling for qualitat ive research. Family Practice 1996; 13: 522-525.The probabil i ty sampling techniques used for quantitat ive studies are rarely appropriatew he n con duc ting qualitat ive research. This art icle considers and explains the differencesbetw een the t w o approaches and describes three broad categories of naturalistic sampling:conve nience , judg em ent and theoretical m odels. The principles are illustrated w ith practicalexamples from the author's own research.Keyword. Qualitat ive sampling.

IntroductionThe benefits of a qualitative approach to health careresearch are becoming increasingly recognized by bothacadem ics and clinicians, but misunderstandings aboutthe philosophical basis and the methodological approachremain. The impression is sometimes given thatqualitative research differs from the hypothetico-deductive model simply in terms of the way that datais collected. The process of sampling is one of the prin-cipal areas of confusion, a problem not helped by theinadequate way that it is covered in the literature, wherether e is little agreement on definitions and authors fre-quently invent new and complex terms which cloudsimp le fundamental is sues . In this article I will describeboth quantitative and qualitative methods of samplingand consider the basic differences between the twoapproa ches in order to explain why the sampling tech-niques used are not transferable. I will consider issuesrelating to sample size and selection in qualitativeresearch and illustrate the principles with practicalexamples.

Quantitative samplingChoosing a study sample is an important step in anyresearch project since it is rarely practical, efficient orethical to study who le populations. Th e aim of all quan-titative sampling approaches is to draw a representativesample from the population, so that the results ofstudying the sample can then be generalized back tothe popu lation. T he selection of an appropriate methoddepends upon the aim of the study. Sometimes lessrigorous methods may be acceptable, such as incidentalor quota samples, but these methods do not guaranteeRece ived 30 May 1996; Accepted 15 July 199 6.Institute of General Practice, University of Exeter, PostgraduateMedical School, Barrack Road, Exeter EX2 5DW, UK.

a representative sample. The most common approachis to use random, or probability samples. In a randomsample the nature of the population is defined and allmembers have an equal chance of selection. Stratifiedrandom sampling and area sampling are variants ofrandom sam pling, which allow subgroups to be studiedin greater detail.Th e size of the sample is determined by the optimumnumber necessary to enable valid inferences to be madeabout the population. The larger the sample size, thesmaller the chance of a random sampling error, but sincethe sampling error is inversely propo rtional to the squareroot of the sam ple size , there is usually little to be gainedfrom studying very large samples. The optimum samplesize depends upon the p arameters of the phenomenon

under study, for example the rarity of the event or theexpected size of differences in outcome between the in-tervention and control groups.

Comparing the quantitative and qualitative approachesThe choice between quantitative and qualitative researchmethods should be determined by the research ques-tion, not by the preference of the researcher. It wouldbe just as inapprop riate to use a clinical trial to examinebehavioural differences in the implementation of clinicalguidelines as it would be to u se participant observationto determine the efficacy of antibiotics for upper respira-tory tract infections. The aim of the quantitative ap-proach is to test ore-determined hypotheses and produ cegeneralizable results. Such studies are useful for a nswer-ing more mechanistic 'what?' questions. Qualitativestudies aim to provide illumination and understandingof complex psychosocial issues and are most usefulfor answering humanistic 'why?' and 'how?' questions.The principal fundamental differences in both thephilosophical foundation of and the methodologicalapproach to the two disciplines are summarized inTable 1.

522

8/2/2019 Qualitative Sampling

2/4

Sampling for qualitative research 523Why is random sampling inappropriate forqualitative studies?The proc ess of selecting a random sam ple is well definedand rigorous, so why can the same technique not beused for naturalistic studies? The answer lies in the aimof the study; studying a random sample provides thebest opportunity to gen eralize the results to the popula-tion but is not the most effective w ay of developing anunderstanding of complex issues relating to humanbehaviour. There are both theoretical and practicalreasons for this.First, samples for qualitative investigations tend tobe small, for reasons explained later in this article. Evenif a representative sample was desirable, the samplingerro r of such a sm all sample is likely to be so large thatbiases are inevitable. Secondly, for a true random sam-ple to be selected, the characteristics under study of thewhole population should be known; this is rarely pos-sible in a complex qualitative study. Thirdly, randomsampling of a population is likely to produce a represen-tative sample only if the research characteristics are n or-mally distributed within the population. There is noevidence that the valu es, beliefs and attitudes that formthe core of qualitative investigation are normallydistributed, making the probability approach inappro-priate. Fourthly, it is well recognized by sociologists1that people are not equally good at observing, und erstan-ding and interpreting their own and other people'sbehaviour. Qualitative researchers recognize that someinformants a re 'rich er' than others and that these peopleare more likely to provide insight and understandingfor the researcher. Choosing someone at random toanswer a qualitative question would be analogous to ran-domly asking a passer-by how to repair a broken downcar, rather than asking a garage mechanicthe formermight have a good stab, but asking the latter is likelyto be more productive.Sample sizeQuantitative researchers often fail to understand theusefulness of studying small samples. This is relatedto the misapprehension that generalizability is theultimate goal of all good research and is the principalreason for some otherwise sound published qualitativestudies containing inappropriate sampling techniques.2An appropriate sample size for a qualitative study isone that adequately answers the research question. Forsimple questions or very detailed studies, this might bein single figures; for complex questions large samplesand a variety of sampling techniques might benecessary. In practice, the number of required subjectsusually becomes obvious as the study progresses, asnew categories, themes or explanations stop emergingfrom the data (data saturation). Clearly this requiresa flexible research design and an iterative, cyclical

approach to samp ling, data collectio n, an alysis and in-terpretation. T his contrasts with the stepwise design ofquantitative studies and makes accurate prediction ofsample size difficult when submitting protocols tofunding bodies.

Sample strategiesThere are three broad approaches to selecting a samplefor a qualitative study.Convenience sampleThis is the least rigorous technique , involving th e selec-tion of the most accessible subjects. It is the least costlyto the researcher, in terms of t ime, effort and money,but may result in poor quality data and lacks intellec-tual credibility. There is an element of conveniencesampling in many qualitative studies, but a morethoughtful approach to selection of a sam ple is usuallyjustified.Judgement sampleAlso known as purposeful samp le, this is the most com-mon sampling technique. The researcher actively selectsthe most productive sample to answer the research ques-tion. This can involve developing a framework of thevariables that might influence an in dividua l's contribu-tion and will be based on the researcher's practicalknowledge of the research ar ea, the available literatureand evidence from the study itself. This is a more in-tellectual strategy than the simple demographicstratification of epidemiological studies, though age,gender and social class might be important variables.If the subjects are known to the researcher, they maybe stratified according to known public attitudes orbeliefs. It may be advantageous to study a broad rangeof subjects (maximum variation sample), outliers(deviant sam ple), subjects who have specific experiences(critical case sample6) or subjects with special exper-tise (key informant sample). Subjects may be able torecommend useful potential candidates for study(snowball sample). During interpretation of the data itis important to consider subjects who support emerg-ing explanations and, perhaps more importantly, sub-jects who disagree (confirming and disconfirmingsamples).Theoretical sampleThe iterative process of qualitative study design meansthat samples are usually theory driven to a greater orlesser extent. Theoretical sampling necessitates buildinginterpretative theories from the emerging data andselecting a new sam ple to examine and elaborate on thistheory. It is the principal strategy for the groundedtheoretical approach3 but will be used in some formin most qualitative investigations necessitatinginterpretation.

8/2/2019 Qualitative Sampling

3/4

524 Family Practicean international journalT AB L E 1 Comparison of quantitative and qualitative methods

PhilosophicalfoundationAimStudy planPosition ofresearcherAssessing qualityof outcomesMeasures ofutility of results

QuantitativeDeductive,reducdonalistT o test pre-sethypothesisStep-wise,predeterminedAims to be detachedand objectiveDirect tests ofvalidity and reliabi-lity using statisticsGeneralizability

QualitativeInductive, holisticT o explore complexhuman issuesIterative, flexibleIntegral part ofresearch processIndirect qualityassurance methods oftrustworthinessT ransferability

It is apparent from the above description that thereis considerable overlap even between these three broadcategories. The relative balance will depend upon theresearch q uestion and the chosen style of data analysisand interpretation. It is important to recognize thatthe essence of the qualitative approach is that it isnaturalisticstudying real people in natural settingsrather than in artificial isolation. Sampling thereforehas to take account not only of the individual'scharacteristics but also temporal, spatial and situationalinfluences, that is, the context of the study. The re-searcher should co nsider the broade r picture: would thisindividual express a different opinion if they were in-terviewed next week or next month? Would they feeldifferently if they we re interviewed at home or at work?Should I study mem when they are under stress or re-laxed? There is no correct answer to these questions,just as there is no perfect way to sample, but the in-fluence that these factors might have on the trustwor-thiness of the results should be acknowledged.A practical examp le of sampling strategyIn practice, qualitative sampling usually requires a flex-ible, pragmatic approach. This may be illustrated bymy o wn study of the professional relationship betweenGPs and specialists (in progress).The way that the two branches of the medicalprofession work together is a key component of theprimary-secondary care interface, which, in terms ofsociological interaction, is largely unresearched. Thestudy aims to describe the current relationship, com-pare this with the historical literature, and elucidate theprincipal factors causing a change in the interaction be-tween the two main branches of the medical profession.Four methods of data collection have been used, eachof which view the interaction from differing perspectivesand each of which have required different samplingstrategies.The first stage involved the use of key informant

interviews,4 an anthropological technique utilizing richinformation sources, which has defined sample selec-tion criteria.5 A sample of 10 national figures in posi-tions of leadership and responsibility within theprofession were chosen. Since the total population ofpossible key informants is small, this was necessarilya convenience sample, though there w as an element ofa judgement approach, since efforts were m ade to ensurethat participants came from a range of clinical,academic, m anagerial and political backgrounds. Theadvantage of this approach lies in its simplicity but itwas difficult to determine at the sampling stage whetherthe informants fulfilled the published selection criteria.The second stage involved in-depth interviews withpractising clinicians throughout the South and WestRegion. T he aim was to develop an understanding andan interpretative framework of the process of interactionbetween specialists and G Ps. I started w ith a judgeme ntsample frame wo rk including variables such as time sincequalification, gender, geographical location, rurality,fundholding status and teaching ho spital status. As thedata was collected and analysed, an interpretativeframework was constructed, so the sampling strategychanged from largely judgement to largely theoretical,in order to build on the developing theory. New themesstopped emerging after about 15 interviews and an ac-ceptable interpretative framework was constructed after24 interviewsthe stage of thematic and theoreticalsaturation.The third stage of the study brought GPs andspecialists together in focus groups to collect the dif-ferent level of data produced by personal interaction.For pragm atic reasons, this had to be conducted in myown locality, and it was important for the study thatthe participants were able to interact in a productive,rather than dysfunctional way. I was able to use mylocal knowledge to satisfy these sampling requirementsusing a combination of convenience and purposivetechniques.The three qualitative stages of the study will formthe basis of a Likert survey to test out emergent themesand which will be distributed to a stratified randomsample of the whole population of clinicians work-ing in the Region. This will represent a different, notnecessarily a stronger, perspective of the professionalrelationship.ConclusionSampling for qualitative research is an area of con-siderable confusion for researchers experienced in thehypothetico-deductive model. This largely relates tomisunderstanding about the aims of the qualitativeapproach, where improved understanding of complexhuman issues is more important than generalizabilityof results. This basic issue explains why probabilis-tic sampling is neither productive nor efficient for

8/2/2019 Qualitative Sampling

4/4

Sampling for qualitative research 525qualitative studies and why alternative strategies areused. Three broad categories of naturalistic samplingtechniques have been describedconvenience, judg e-ment and theoretical samplingthough in practice thereis often considerable overlap between these approaches.AcknowledgementsI would like to thank Ms Nicky Britten for her com-ments on this paper. T he study of the professional rela-tionship between specialists and GPs is funded by aResearch and Training Fellowship from the Researchand Development Directorate of the South and WestRegional Health Authority.

References1 Jackson JA. Professions and professionalisation. Cambridge:Cambridge University Press, 1970.2 Pound P, Bury M, Gompertz P, Ebrahim S. Stroke patients'views on their admission to hospital. Br MedJ 1995; 31 1:1&-22.3 Glaser BG, Strauss AL. The discovery of grounded theory:

Strategies for qualitative research. London: W eidenfield andNicholson, 1968.4 Marshall MN. The key informant technique. Fam Pract 1996;13 : 92- 97 .5 Burgess RG (ed.). Field research: a sourcebook and manual.London: Routledge, 1989.6 Bradley C. Turning anecdotes into datathe critical incidenttechnique. Fam Pract 1992; 9: 98-103.