Sampling in qualitative research

Plan of presentation

Sampling introductionPre requisite of samplingTypes of samplingQualitative research introductionSampling strategiesAdvantages and disadvantagesSummary References

Sampling

Sample: A subset of the population being studied from which data is actually collected.

Sampling frame: This is the actual list of sampling units from which the sample, or some stage of the sample, is selected. It is simply a list

of the study population.

Contd.

Sample design: This refers to a set of rules or procedures

that specify how a sample is to be selected.

Sample size: The number of elements in the obtained

sample.

Sampling : The processes by which the subset of the

population from which you will collect data are chosen

Pre requisites of sampling

What are the research objectives? What is the target population?Who should be inclusion criteriaWho should be exclusion criteriaWhat 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?

Contd.

How are the data to be analysed? What data collection methods should be employed? How long will the interview be? Sample size? Sampling frame?How should potential respondents/participants be recruited?

Types of sampling

Probabilistic : A sampling method where the probability of selection of population elements is known.

Non-probabilistic :The process of selecting a sample from a population without using (statistical) probability theory.

Mixed

What a qualitative researcher worries about

Qualitative research

When the paramount objective is “understandingWhen variables cannot be quantifiedWhen variables are best understood in their natural settingsWhen studying intimate details

of roles, processes, and groups

Sampling strategies

Purposive or Judgemental Sampling

A. Sampling to Achieve Representativeness or Comparability

B. Sampling Special or Unique Cases

C. Sequential Sampling

D. Sampling Using Multiple Purposive Techniques

Convenience Sampling

A. Captive Sample

B. Volunteer Sample

Others

Stakeholder

Paradigmatic

Purposeful Random Sampling

Stratified Purposeful Sampling

Expert sampling

Quota sampling

I- Sampling to achieve representativeness or comparability

These techniques are used when the researcher wants to :

(a) select a purposive sample that represents a broader group of cases as closely as possible or

(b) set up comparisons among different types of cases

a)Typical case sampling or modal instance

Focuses on what is typical, normal, and/or average. This

strategy may be adopted when one needs to present a

qualitative profile of one or more typical cases.

When using this strategy you must have a broad

consensus about what is “average.”

Ex:typical voter

b)Extreme or deviant case sampling

Looks at highly unusual manifestations of the

phenomenon of interest, such as outstanding

success/notable failures, top of the class/dropouts, exotic

events, crises.

This strategy tries to select particular cases that would

glean the most information, given the research question

c)Intensity sampling

Chooses information-rich cases that manifest the

phenomenon intensely, but not extremely, such as good

students/poor students, above average/below average.

This type of sampling requires that you have prior

information on the variation of the phenomena.

Ex:jeolousy reaction

d)Maximum variation sampling

Selects a wide range of variation on dimensions of interest. The purpose is to discover/uncover central themes, core elements, and/or shared dimensions that cut across a diverse sample while at the same time offering the opportunity to document unique or diverse variations.

Aka phenomenal variation sampling.

e)Homogenous sampling

Brings together people of similar backgrounds and experiences. It reduces variation, simplifies analysis, and facilitates group interviewing.

This strategy is used most often when conducting focus groups.

f)Reputational case sampling

Variant of snowball samplingEase of identifying the first case.Example :traffic signal beggars.

II- Sampling special or unique cases

Employed when the individual case itself, or a specific

group of cases, is a major focus of the investigation

(rather than an issue).

a)Revelatory case sampling

It involves identifying and gaining entry to a single case representing a phenomenon that had previously been ‘inaccessible to scientific investigation’’.

Such cases are rare and difficult to study, yet yield very valuable information about heretofore unstudied phenomena.

b)Critical case sampling

Looks at cases that will produce critical information. In

order to use this method, you must know what constitutes

a critical case.

This method permits logical generalization and maximum

application of information to other cases because if it's

true of this one case, it's likely to be true of all other case.

Ex: federal rules and lawyers

c)Sampling politically important cases

Seeks cases that will increase the usefulness and

relevance of information gained based on the politics of

the moment.

This strategy attracts attention to the study (or avoids

attracting undesired attention by purposefully eliminating

from the sample politically sensitive cases). This strategy

is a variation on critical case sampling.

d)Criterion sampling

Selects all cases that meet some criterion.

This strategy is typically applied when considering

quality assurance issues. In essence, you choose cases that

are information-rich and that might reveal a major system

weakness that could be improved.

Ex:hospital stay.

III- Sequential sampling

Uses the gradual selection principle of sampling when

(a) the goal of the research project is the generation of

theory (or broadly defined themes) or

(b) the sample evolves of its own accord as data are being

collected.

a)Theoretical or operational construct sampling

Identifies manifestations of a theoretical construct of

interest so as to elaborate and examine the construct.

This strategy is used in grounded theory studies.

You would sample people/incidents, etc., based on

whether or not they manifest/represent an important

theoretical or operational construct.

b)Confirming or disconfirming cases

Seeks cases that are both “expected” and the “exception” to what is

expected. In this way, this strategy deepens initial analysis, seeks

exceptions, and tests variation.

In this strategy you find both confirming cases (those that add depth,

richness, credibility) as well as disconfirming cases (example that do not

fit and are the source of rival interpretations).

This strategy is typically adopted after initial fieldwork has established

what a confirming case would be. Ex: academics &environment.

c)Opportunistic or emergent sampling

Follows new leads during fieldwork, takes advantage of

the unexpected, and is flexible. This strategy takes

advantage of whatever unfolds as it is unfolding, and may

be used after fieldwork has begun and as a researcher

becomes open to sampling a group or person they may not

have initially planned to interview.

d)Snowball sampling

This is an approach used for locating information-rich cases.

Used mostly to reach out to “hard to find population”Nominated ,chain- referral, link tracing, network

sampling

Advantages & Disadvantages

Simple,cost-efficient,enables access to hard to find population.

Works well when members know each other.

Cons:

Fate of study depend on first recruitment

Cant control number of members recruited

Misses out on the isolated ones

Representativeness??

Interviewer bias.

Solutions

Key informant sampling:

Involves speaking to knowledgeable person.eg. Rehab

centre councellor .

Target sampling: done in 2 stages

a) Map the target population

b) Recruit pre-specified numbers of participants from each

site identified by ethnomapping.

IV- Combination or Mixed Purposeful Sampling

Combines two or more strategies listed above.

This type of sampling meets multiple interests and needs.

Convenient, accidental or haphazard Sampling

A. Captive SampleB. Volunteer Sample

Convenient sampling

Selects cases based on ease of accessibility. Argument: Equivalent to simple random sampling.Eg :people in street interview.

Advantages of convenient sampling

This strategy saves time, money, and effort(quick,low cost

hassle free,no rules)

In pilot studies,gives data and trends of pop. without

complication of using a randomized sample.

Disadvantages of convenient sampling

Not representative of the population which leads to low external validity.

Systematic bias : This refers to a constant difference between the results from

the sample and the theoretical results from the entire population. This stems

from over or under representation which leads to skewed results.

Because the probability of inclusion in the sample is unknown for each

respondent, none of the reliability or sampling precision statistics can be

calculated.

This strategy may yield information-poor cases

Volunteer sampling

There is no fixed line of differenceThere is no sampling framePeople are self motivated and volunteer

Disadvantages of volunteer sampling

Result of study totally depends on people who

volunteered

Cant compare as it cant be determined that those who did

not volunteer , to what extent differed from those who

volunteered.

Purposeful Random Sampling

This strategy adds credibility to a sample when the

potential purposeful sample is larger than one can handle.

It uses small sample sizes, thus the goal is credibility, not

representativeness or the ability to generalize.

Ex:select 10 out of 300 drug addicts in rehab.

Stratified Purposeful Sampling

Focuses on characteristics of particular subgroups of

interest; facilitates comparisons.

This strategy is similar to stratified random sampling

(samples are taken within samples), except the sample

size is typically much smaller.

In stratified sampling you “stratify” a sample based on a

characteristic.

The main goal of this strategy is to capture major

variations (although common themes may emerge).

Paradigmatic Case Sampling

A case is “paradigmatic” when it is considered the

exemplar for a certain class.

For example, if one wanted to study the management of

professional sports teams, the paradigmatic case in

hockey of a successful franchise would be the Montreal

Canadians; for baseball it would be the New York

Yankees.

Stakeholder Sampling

Particularly useful in the context of evaluation research

and policy analysis, this strategy involves identifying who

the major stakeholders are who are involved in designing,

giving, receiving, or administering the programme or

service being evaluated, and who might otherwise be

affected by it.

Expert sampling

Expert sampling involves the assembling of a sample of persons with

known or demonstrable experience and expertise in some area. Often,

we convene such a sample under the auspices of a "panel of experts."

There are actually two reasons you might do expert sampling.

First, to get the the views of persons who have specific expertise. In

this case, expert sampling is essentially just a specific subcase of

purposive sampling.

Second: Evidence of validity.

Contd.

Advantage:The advantage of doing this is that you aren't out on your

own trying to defend your decisions -- you have some acknowledged experts to back you

Disadvantage:The disadvantage is that even the experts can be, and

often are, wrong

Quota sampling

Non-probability version of stratified sampling

Characteristics chosen are mutually exclusive and isolated

prior to sampling

A quota is fixed,no randomization,no list used.

May or may not be representative of population.

Quota types

Proportional &non-proportionalInterlocked and non interlocked.

Advantages and disadvantages

Pros :Quick and cheapAllows study of traits and characteristics of subgroups

and also the relation between themCons :Introduction of unknown biasDifficult to choose the variables as some may be highly

redundant.

Famous mistake of quota sampling

How do purposive and quota sampling differ?

Purposive and quota sampling are similar in that they both seek to identify participants based on selected criteria.

Quota sampling is more specific with respect to sizes and proportions of subsamples, with subgroups chosen to reflect corresponding proportions in the population.

Studies employ purposive rather than quota sampling when the number of participants is more of a target than a steadfast requirement – that is, an approximate rather than a strict quota.

Biases

Sampling biasSystematic biasInterviewers biasObservers bias.Ethical consideration .

Summary

Sampling: a)Probabilistic

b)Non-probabilisticNon-probabilistic sampling used in qualitative research .Broadly categorized into: Purposive

convenient

References

From Lisa M. Given (Ed.) (2008). The Sage Encyclopedia of Qualitative Research Methods. Sage: Thousand Oaks,CA, Vol.2, pp.697‐698.

K. Sabin, HIV Department, World Health Organisation, 20 avenue Appia, Geneva 27, Switzerland.

SOCIOLOGICAL METHODS & RESEARCH, Vol. 10 No. 2. November 1981 141-163 Social Problems, Vol .44,No.2 May 1997 http://hdl.handle.net/10125/227 Ethnobotany Research & Applications 5:147-158 (2007) Discussion Draft: Considerations and Recommendations Regarding the Use of

Judgmental Sampling in Soil Investigations Krista J. Gile and Mark S. Handcock. Respondent-driven sampling: An assessment of

current methodology. Sociological Methodology, 40:285–327,2010. URL http://arxiv.org/abs/0904.1855v1.

Journal of Mixed Methods Research 2007; 1; 77Charles Teddlie and Fen Yu Respondent driven sampling :A new approch to study hidden population,Heckathon.

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