Snowball Sampling: A Purposeful Method of Sampling in Qualitative Research

Strides Dev Med Educ. 14(3):e67670.

Published online 2017 September 30.

doi: 10.5812/sdme.67670.

Research Article

Snowball Sampling: A Purposeful Method of Sampling in Qualitative

Research

Mahin Naderifar,

Hamideh Goli,

and Fereshteh Ghaljaie

1,*

Department of Nursing, Faculty of Nursing and Midwifery, Zabol University of Medical Sciences, Zabol, IR Iran

Corresponding author: Fereshteh Ghaljaie, Department of Nursing, Faculty of Nursing and Midwifery, Zabol University of Medical Sciences, Zabol, IR Iran, E-mail:

[email protected]

Received 2016 November 29; Revised 2017 May 03; Accepted 2017 June 25.

Abstract

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

1. Introduction

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,

stratiﬁed sampling, and cluster sampling.

On the other hand, nonprobability methods of sam-

pling involve samples that are available to the researcher

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Naderifar M et al.

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-

inals) (4).

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-

ball (5).

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-

search (8).

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

sampling method.

2. Methods

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-

scriptive review.”

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

required information, including the title, location, time,

scope, and method of sampling; this checklist was used for

2 Strides Dev Med Educ. 14(3):e67670.

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Naderifar M et al.

the ﬁnal evaluation. Two individuals separately searched

and gathered the data.

3. Results

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-

ble 1).

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-

ferent aspects.

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-

get population.

Adaptations of snowball sampling strategies shouldbe

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-

tion (9).

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-

searcher (11).

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

Supplementary material(s) is available here [To read

supplementary materials, please refer to the journal web-

site and open PDF/HTML].

Acknowledgments

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-

tion.

References

1. Abedsaeidi J, Amiraliakbari S. Research Method in Medical Sciences and

health. Tehran: Salemi; 2015.

Strides Dev Med Educ. 14(3):e67670. 3

Uncorrected Proof

Naderifar M et al.

2. Polit DF, Beck CT. Essentials of nursing research: Appraising evidence for

nursing practice. Lippincott Williams Wilkins; 2010.

3. Speziale HS, Streubert HJ, Carpenter DR. Qualitative research in nurs-

ing: Advancing the humanistic imperative. Lippincott Williams Wilkins;

2011.

4. Hejazi S. Sampling anditsvariants: Introduction to Research Methodology

in Medical Sciences. Tehran: Islamic Azad University; 2006.

5. Ahmadzadehasl M, Ariasepehr S. Sampling and samplesize calculation:

Basic Principles of Research in Medical Sciences. Tehran: Nour Danesh;

2010.

6. Burns N, Grove SK. The practice of nursing research. Conduct Critique

Util. 1993;4.

7. Polit-O’Hara D, Beck CT. Essentials of nursing research: Methods, ap-

praisal, and utilization. 1. Lippincott Williams Wilkins; 2006.

8. Jalali R. qqualitative research sampling. J Qual Res Health Sci.

2013;1(4):310–20.

9. Sadler GR, Lee HC, Lim RS, Fullerton J. Recruitment of hard-to-

reach population subgroups via adaptations of the snowball sam-

pling strategy. Nurs Health Sci. 2010;12(3):369–74. doi: 10.1111/j.1442-

2018.2010.00541.x. [PubMed: 20727089].

10. Magnani R, Sabin K, Saidel T, Heckathorn D. Review of sampling hard-

to-reach and hidden populations for HIV surveillance. AIDS. 2005;19

Suppl 2:S67–72. [PubMed: 15930843].

11. Hughes M. Buzzmarketing: get people to talk about your stuﬀ. Penguin;

2005.

12. Walsh A, Edwards H, Fraser J. Attitudes and subjective norms: deter-

minants of parents’ intentions to reduce childhood fever with med-

ications. Health Educ Res. 2009;24(3):531–45. doi: 10.1093/her/cyn055.

[PubMed: 18974070].

13. Charkazi A, Heshmati H, Neirizi O. Explaining smoking among stu-

dents at Golestan University of Medical Sciences based on basnef

model. J Health Syst Res. 2012;7(6):986–93.

14. Heshmati F, Vanaki Z, Mohammadi I. Barrier to forming and implement-

ing academic service partnership in nursing; a qualitative study. 2010.

15. Swallow VM, Allen D, Williams J, Smith T, Crosier J, Lambert H, et al.

Pan-Britain, mixed-methods study of multidisciplinary teams teach-

ing parents to manage children’s long-term kidney conditions at

home: study protocol. BMC Health ServRes. 2012;12:33. doi: 10.1186/1472-

6963-12-33. [PubMed: 22333296].

16. Sasson C, Forman J, Krass D, Macy M, Kellermann AL, McNally BF. A

Qualitative Study to Identify Barriers to Local Implementation of Pre-

hospital Termination of Resuscitation Protocols. Circulat Cardiovasc

Qual Outcomes. 2009;2(4):361–8. doi: 10.1161/circoutcomes.108.830398.

17. Zaghloul AA, Alsokair MK. Constructing a nurse appraisal form: A

Delphi technique study. J Multidiscip Healthc. 2008;1:1–14. [PubMed:

21197327].

18. Zareipour MA, Sadeghi R, SadeghiTabatabaei SA, Seyedi S.Eﬀective fac-

tors on smoking based on basnef model in male students in tehran

medical sciences university in 2009. J Urmia Nurs Midwifery Faculty.

2011;9(1).

19. Lagu T, Kaufman EJ, Asch DA, Armstrong K. Content of weblogs writ-

ten by health professionals. J Gen Intern Med. 2008;23(10):1642–6. doi:

10.1007/s11606-008-0726-6. [PubMed: 18649110].

20. Schreiber RS, MacDonald MA. Keeping vigil over the profession: a

grounded theory of the context of nurse anaesthesia practice. BMC

Nurs. 2010;9:13. doi: 10.1186/1472-6955-9-13. [PubMed: 20633286].

21. Mishima SM, Pereira FH, MatumotoS, Fortuna CM, Pereira MJ,Campos

AC, et al. Assistance in family health from the perspective of users. Rev

Lat Am Enfermagem. 2010;18(3):436–43. [PubMed: 20721434].

22. Cataldo JK, Malone RE. False promises: the tobacco industry, "low tar"

cigarettes, and older smokers. JAm Geriatr Soc. 2008;56(9):1716–23. doi:

10.1111/j.1532-5415.2008.01850.x. [PubMed: 18691279].

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Naderifar M et al.

Table 1. 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

medications

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

clinical practice.

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

protocol

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

invited.

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

protocols.

Zaghloul and Alsokair (17) Constructing a nurse appraisal form: A

Delphi technique study

Delphi technique and snowball

sampling method

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

recruited.

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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

2009

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

peers.

Lagu et al. (19) Content of weblogs written by health

professionals

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

anesthesia practice

Purposeful and snowball sampling

methods

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

levels.

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

unsuccessful.

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.

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