Strides Dev Med Educ. 2017 September; 14(3):e67670.
Published online 2017 September 30.
Snowball Sampling: A Purposeful Method of Sampling in Qualitative
Mahin Naderifar,1Hamideh Goli,2and Fereshteh Ghaljaie3,*
1Department of Nursing, Faculty of Nursing and Midwifery, Zabol University of Medical Sciences, Zabol, IR Iran
2Department of Nursing, Faculty of Nursing and Midwifery, Zahedan University of Medical Sciences, Zahedan, Iran
3Community Nursing Research Center, Faculty of Nursing and Midwifery,Zahedan University of Medical Sciences, Zahedan, Iran
*Corresponding author: Fereshteh Ghaljaie, Community Nursing Research Center, Faculty of Nursing and Midwifery, Zahedan University of Medical Sciences, Zahedan, Iran.
Received 2016 November 29; Revised 2017 May 03; Accepted 2017 June 25.
Background and Objectives: Snowball sampling is applied when samples with the target characteristics are not easily accessible.
This research describes snowball sampling as a purposeful method of data collection in qualitative research.
Methods: This paper is a descriptive review of previous research papers. Data were gathered using English keywords, including “re-
view,” “declaration,” “snowball,” and “chain referral,” as well as Persian keywords that are equivalents of the following: “purposeful
sampling,” “snowball,” “qualitative research,” and “descriptive review.” The databases included Google Scholar,Scopus, Irandoc, Pro-
Quest, Science Direct, SID, MagIran, Medline, and Cochrane. The search was limited to Persian and English articles written between
2005 and 2013.
Results: The preliminary search yielded 433 articles from PubMed, 88 articles from Scopus, 1 article from SID, and 18 articles from
MagIran. Among 125 articles, methodological and non-research articles were omitted. Finally, 11 relevant articles, which met the
criteria, were selected for review.
Conclusions: Diﬀerent methods of snowball sampling can be applied to facilitate scientiﬁc research, provide community-based
data, and hold health educational programs. Snowball sampling can be eﬀectively used to analyze vulnerable groups or individ-
uals under special care. In fact, it allows researchers to access susceptible populations. Thus, it is suggested to consider snowball
sampling strategies while working with the attendees of educational programs or samples of research studies.
Keywords: Purposeful Sampling, Snowball, Qualitative Research, Descriptive Review
Qualitative research is an organized method of de-
scribing people’s experiences and internal feelings (1). It
can be said that qualitative research provides a thorough
and deep overview of a phenomenon through data col-
lection and presents a rich description using a ﬂexible
method of research. In this method, qualitative informa-
tion, which is gathered in the form of non-numerical data,
is presented (2).
There are diﬀerent methods to collect the required
data, including interviews, observations, focus groups,
narratives, notes, reports, and review of archives. The
researcher chooses the information with respect to the
questions, sensitivity of the subject, research samples, and
availability of resources (3). To determine the characteris-
tics of a community, it is possible to gather data by sam-
pling or census reports.
Sampling is the process of choosing a part of the popu-
lation to represent the whole. If the researcher considers a
part of the population as a representation of the whole, the
analysis will be more comprehensive (4). In many research
studies, factors such as lack of human resources, lack of
precision, high expenses, inadequate equipment, and pop-
ulation dispersion prevent researchers from studying the
entire population. In this case, it is preferable to study only
a part of the population (5).
Researchers should plan the sampling process and de-
termine the method of study. Sampling is performed in
two general ways: probability and nonprobability. In prob-
ability sampling methods, the rules of probability are ap-
plied, and as their main feature, each sample has a chance
to be selected. In these methods, the researcher’s opin-
ion or community members do not inﬂuence the selection
of samples. The selected sample is a representative of the
population, and the researcher can generalize the ﬁndings
to the whole population. Probability sampling methods
include simple random sampling, systematic sampling,
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Naderifar M et al.
stratiﬁed sampling, and cluster sampling.
On the other hand, nonprobability methods of sam-
pling involve samples that are available to the researcher
or are selected by the researcher. In these methods, not ev-
eryone has an equal chance of being selected, and it is not
clear who will be included in the ﬁnal sample. Regarding
the sampling method, generalization of the ﬁndings to the
entire population is not clear either, and one cannot calcu-
late the rate of error in the sampling. Diﬀerent methods of
nonprobability sampling include convenience, purpose-
ful, and quota sampling. In quantitative research, proba-
bility sampling is normally applied, whereas in qualitative
research, nonprobability sampling is selected (1).
The convenience sampling method includes members
of the population who are available to the researcher. For
instance, a lecturer who distributes questionnaires among
students has in fact used this method. Asking questions
from passers-by on the street is another example of this
method of sampling. This method is also called “acciden-
tal sampling” (1). Snowball sampling is a convenience sam-
pling method. This method is applied when it is diﬃ-
cult to access subjects with the target characteristics. In
this method, the existing study subjects recruit future sub-
jects among their acquaintances. Sampling continues un-
til data saturation (6).
As stated by Polit-O’Hara and Beck, this method, which
is also called the “chain method,” is eﬃcient and cost-
eﬀective to access people who would otherwise be very
diﬃcult to ﬁnd. In this method, the researcher asks the
ﬁrst few samples, who are usually selected via convenience
sampling, if they know anyone with similar views or situ-
ations to take part in the research. The snowball method
not only takes little time but also provides the researcher
with the opportunity to communicate better with the sam-
ples, as they are acquaintances of the ﬁrst sample, and the
ﬁrst sample is linked to the researcher (7). This type of net-
working is particularly useful for ﬁnding people who are
not willing to reveal their identities (e.g., addicts and crim-
In another deﬁnition, snowball sampling may be less
reliant on a reference sample, but it is still suitable to ﬁnd
unattainable populations. For example, when the research
is aimed at a group of illegal immigrants or addicts, meet-
ing the ﬁrst group of samples will probably lead to other
samples; thus, the study sample grows like a rolling snow-
Generally, snowball sampling is a gradual process, and
time inﬂuences the selection of samples. Sampling usu-
ally continues until data saturation. On the other hand,
convenience sampling is the weakest method of sampling.
The risk of bias is low when the population is homoge-
neous in terms of the target characteristic under question,
whereas in nonhomogeneous populations, this method of
sampling has a higher risk of error (1).
In recent decades, qualitative research has become
more popular in nursing studies. Despite the growing
body of qualitative research in the past few decades, there
have been debates about these types of studies due to lack
of detailed information on the methods and processes.
Most published qualitative papers do not provide enough
information about the characteristics of the samples, re-
search, and sampling methods. In qualitative research,
sampling is determined by the type of research, while most
published literature has not determined the type of re-
Consecutive sampling is one method of purposeful
sampling in qualitative research. In this method, instead
of selecting a ﬁxed sample, every subject who meets the cri-
teria is selected until the required sample size is achieved.
This method is classiﬁed into three types, one of which is
snowball sampling. This type of sampling is a nonprob-
ability method, which involves random selection of sub-
jects. This method is most eﬀective when the members
of the population are not easily accessible (e.g., homeless
people, illegal immigrants, and addicts). The researcher
ﬁrst identiﬁes a group of people, and after gathering data,
he/she asks them to recommend similar cases for the study.
The purpose of qualitative research is to gain a deeper
understanding of a phenomenon, rather than to general-
izing the ﬁndings. Therefore, careful selection of research
samples can help us conduct a more thorough evaluation.
The purpose of this study was to review the available qual-
itative research in nursing, which applied the snowball
This research is a thorough and descriptive review of
the snowball sampling method, based on articles pub-
lished in national and international journals, as well as dis-
sertations. The articles were selected from Cochrane, Pro-
Quest, Science Direct, SID, MagIran, Medline, Irandoc, Sco-
pus, and Google Scholar databases in both Persian and En-
glish languages between 2005 and 2013. Data were gath-
ered using English keywords, including “review,” “declara-
tion,” “snowball,” and “chain referral,” as well as Persian
keywords that are equivalents of the following: “purpose-
ful sampling,” “snowball,” “qualitative research,” and “de-
First, all articles related to qualitative research in Iran
were gathered. The articles, which contained the afore-
mentioned keywords in the abstract, were included in our
preliminary list, while the rest of the articles were dis-
carded. Then, a checklist was used to document all the
2Strides Dev Med Educ. 2017; 14(3):e67670.
Naderifar M et al.
required information, including the title, location, time,
scope, and method of sampling; this checklist was used for
the ﬁnal evaluation. Two individuals separately searched
and gathered the data.
In the preliminary search, 423 articles from PubMed,
88 articles from Scopus, 1 article from SID, and 18 articles
from MagIran were selected. Dissertations approved be-
tween 2005 and 2013 were also reviewed. After limiting the
search to articles with full text, the total number of the ar-
ticles reached 125. In addition, dissertations that were not
available to the researchers were eliminated.
The remaining articles were reviewed, and those with
a theoretical framework, as well as non-research articles,
were omitted. Finally, 11 articles were found eligible for the
review and were selected for their relevance to the purpose
of this study. The methods and content of these papers
were thoroughly reviewed to determine the characteristics
of snowball sampling and methods of implementation (Ta-
4. Discussion and Conclusions
One main challenge of qualitative research is selecting
the samples, determining the suﬃcient sample size, and
explaining the sampling procedure. Although many quali-
tative research experts believe that data saturation is a sub-
jective phenomenon, there is also the view that more ob-
servations and interviews do not aﬀect the interpretation
of the results.
Review of the literature showed that a few research
studies have accurately explained the snowball sampling
method. In most of them, the main method of sampling
was suﬃcient, and they have not presented clear expla-
nations about its implementation. On the other hand, in
some studies, the snowball sampling method did not suf-
ﬁce and was only used to complete the data. It should be
mentioned that the snowball sampling method is fully ex-
plained in only one study, which has fully clariﬁed its dif-
Diﬀerent variations of snowball sampling can be ap-
plied in the development of community-based informa-
tion, dissemination eﬀorts related to health education pro-
grams, and research studies. These methods can be eﬀec-
tively used to choose samples from fragile populations or
people under specialized care. Adaptation of the snowball
sampling method helps researchers gain access to the tar-
Adaptations of snowball sampling strategies should be
considered when recruiting participants for educational
programs or research studies. In snowball sampling, the
fragile population is selected in a social context and in a
multi-stage process. After gaining access to the prelimi-
nary samples, the samples begin to introduce other people
to take part in the research. This process will continue in
a semi-automatic and chain-like manner until data satura-
Nurses have diﬀerent responsibilities, including clini-
cal care, education, and research. In many cases, it is dif-
ﬁcult to identify or contact care-seekers. Human immun-
odeﬁciency virus (HIV) positive patients, abused women,
drug addicts, sex workers, and people with homosexual
or asexual tendencies are examples of these fragile popu-
lations (10). The snowball sampling method has been de-
rived from diﬀerent concepts of social marketing. How-
ever, people who are involved in research studies and have
educational opportunities should always consider individ-
ual rights and privacy.
Snowball sampling is a method of gathering informa-
tion to access speciﬁc groups of people. The advantages
and limitations of this research method should be eval-
uated to select the best strategy. A researcher aiming to
perform health interventions should consider people’s pri-
vacy concerns (both for preliminary samples and samples
in the target community). Additionally, anonymity and
conﬁdentiality of the data should be guaranteed by the re-
The limitations of this study, which are mostly related
to its design (review study), include lack of homogeneity in
the measurement methods and lack of explanation about
the exact sampling method in most papers. Researchers
interested in qualitative research are recommended to
choose a method of sampling that is not only more accu-
rate but also saves time and money.
Supplementary material(s) is available here [To read
supplementary materials, please refer to the journal web-
site and open PDF/HTML].
The researchers would like to thank our research col-
leagues in Zabol University of Medical Sciences and Za-
hedan University of Medical Sciences, for their coopera-
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Naderifar M et al.
Table1. General Data Gathered from the Selected Articles on the Snowball Sampling Method
Reference Title Data Collection Method Findings
Walsh et al. (12)Attitudes and subjective norms:
Determinants of parents’ intentions to
reduce childhood fever with
Three methods: newspaper
advertisement, face-to-face interview,
and snowball sampling
The snowball method was implemented
by a network of researchers and
respondents. All the attendees were
given a package containing an
introduction letter, information
overview sheet, questionnaire, and
return envelope. After completing the
forms, the information was collected.
Charkazi et al. (13)Explaining smoking among students at
Golestan University of Medical Sciences
based on BASNEF model
Snowball sampling (The researcher
identiﬁed some smoking students, and
after explaining the purpose of the study
and obtaining oral consent, they were
asked to introduce some other smoker
students to recruit a larger sample.)
Abstract norms and enabling factors are
among the most important factors in
smoking. Therefore, these points should
be considered for preventing and
organizing recovery programs.
Heshmati Nabavi et al. (14)Barrier to forming and implementing
academic service partnership in
nursing: A qualitative study
Purposeful sampling, snowball
sampling, and interview (First,
purposeful sampling was applied, and
then more samples were identiﬁed
using the snowball method. This process
continued until data saturation.)
The identiﬁed themes were
“organizational divergence,” “invisible
wall,” and “overemphasis on theoretical
knowledge.” The most important
barriers impeding cooperation between
educational and clinical institutes are
ambiguities in the organizational
structure of institutes, responsibilities
of employees of each institute in
relation to others, overemphasis on
theoretical knowledge, and ignoring of
practical knowledge and clinical
performance of nurses. Development of
academic service partnership can be
facilitated by deﬁning a formal
organizational relationship between
academic and service institutes,
clarifying roles and responsibilities of
each institute regarding clinical
education, deﬁning clinical practice
roles for nursing faculty members, and
emphasizing the importance of nursing
Swallow et al. (15)Pan-Britain, mixed-methods study of
multidisciplinary teams teaching
parents to manage children’s long-term
kidney conditions at home: Study
Purposeful sampling was applied
considering the child’s age, diagnosis,
ethnicity,and need for clinical care.
Snowball sampling was implemented to
identify staﬀ members who were
involved in the care of the child.
It is essential to educate the parents on
long-term care for children at home. It is
also essential to understand that parents
of children with chronic renal diseases
need to receive specialized education.
Sasson et al. (16)A qualitative study to identify barriers to
local implementation of pre-hospital
termination of resuscitation protocols
First, a focus group was determined, and
then using snowball sampling, more
samples entered the study. Emergency
room doctors, paramedics, emergency
room authorities, and nurses were also
A method Focus group was used instead
of a response package to have a better
understanding of how emergency
systems manage resuscitation protocols
and which practical factors impede the
implementation of resuscitation
Zaghloul and Alsokair (17)Constructing a nurse appraisal form: A
Delphi technique study
Delphi technique and snowball
A standardized nurse appraisal form was
designed and implemented in the
hospitals of the eastern states of Saudi
Arabia, which are covered by health
organizations. The nurses’ ideas about
the appraisal form were used to
determine diﬀerent aspects of a nurse’s
performance. In each hospital, the ﬁrst
head nurse was introduced to other
head nurses to participate in the study.
This process continued until enough
head nurses and supervisors were
Strides Dev Med Educ. 2017; 14(3):e67670. 5
Naderifar M et al.
Zareipour et al. (18)Eﬀective factors on smoking based on
BASNEF model in male students in
Tehran Medical Sciences University in
Snowball sampling method (The
researcher met a smoking person, and
after attaining oral consent, he was
asked to complete a questionnaire.
Then, he was asked to invite other
smoker friends to take part in the study.
The researcher recruited the samples
and collected the questionnaires.)
Based on this model, the importance of
normative, enabling, and observational
factors in smoking behaviors was
determined, and the need for
authorities’ attention was highlighted.
Improvement of social and life skills,
including resistance against peer
pressure (saying “no”), and of the sense
of responsibility and self-conﬁdence in
the youth can be eﬀective. The youth
smoke out of fear of humiliation and
shame. However, through building
conﬁdence, they can inﬂuence their
Lagu et al. (19)Content of weblogs written by health
Modiﬁed snowball sampling method The domain and content of medical
weblogs were reviewed to determine
how much the writer of these posts had
revealed facts about the patient,
breached the doctor-patient
conﬁdentiality,or outlined the
shortcomings of care for the patient. The
researchers deﬁned medical weblogs as
web pages, which have medical content
and are written by healthcare
professionals. Weblogs are a developing
part of the medical profession’s public
face. In these weblogs, doctors and
nurses can share their opinions.
However, they can also jeopardize their
career by revealing conﬁdential
information in their content.
Schreiber and MacDonald (20)Keeping vigil over the profession: A
grounded theory of the context of nurse
Purposeful and snowball sampling
Management of registered anesthesia
nurses in diﬀerent cultural-political
backgrounds of patient care was
studied. The grounded theory was used
to observe the performance of
registered and certiﬁed anesthetic
nurses. The grounded theory was used
to ﬁnd procedures in diﬀerent social
backgrounds. The grounded theory was
used since the researchers were
interested to show the process using
which the nurses performed their role.
Mishima et al. (21)Assistance in family health from the
perspective of users
Snowball sampling method (chain
sampling) according to the location of
the original interviewed group that met
the inclusion criteria
Family health services were evaluated
from the perspective of clients in San
Paolo and Riviera Porto, Brazil, in a wide
range of primary healthcare services
and specialized care services at diﬀerent
Cataldo and Malone (22)False promises: The tobacco industry,
“low-tar” cigarettes, and older smokers
Snowball sampling The role of tobacco industries in
marketing and damage from tobacco
addiction were assessed among old
smokers and infants. The researcher
sought to ﬁnd documents from the
tobacco industry,which was
Sadler et al. (9)Recruiting hard-to-reach United States
population sub-groups via adaptations
of snowball sampling strategy
Snowball sampling to recruit subgroups
that were not easily accessible
Variations of the snowball sampling
strategy can be applied in the
development of community-based
information, health education
programs, and research studies. This
strategy is eﬀective in enlisting the
involvement of members from
vulnerable populations. These strategies
ﬁnd individuals, who have the desired
characteristics, and use that person’s
social networks to recruit similar
subjects in a multi-stage process. This
semi-automatic process continues until
enough samples have been recruited.
6Strides Dev Med Educ. 2017; 14(3):e67670.