Position Statement: Ethical Considerations in Snowball Sampling

Considerations for a common approach for recruitment and data collection.

Introduction

Snowball sampling is an approach to data collection where existing research and evaluation participants refer potential participants from their networks. It's a widely used recruitment strategy, and its efficiency in reaching hard-to-access populations and reducing recruitment costs makes it an attractive methodological choice. However, this recruitment approach carries distinct ethical considerations that warrant careful attention.

While snowball sampling can be consistent with the principles of the National Statement on Ethical Conduct of Human Research 2025, its implementation requires thoughtful design to ensure that the rights, privacy, and autonomy of potential participants are appropriately protected.

Through our review of applications across market research, evaluation, and social research contexts, we have observed significant variation in how snowball sampling is designed and implemented. Some applications demonstrate rigorous attention to privacy and consent, while others inadvertently create conditions where potential participants' information is shared without their knowledge or consent, and where social dynamics may compromise voluntary participation.

This position statement articulates our perspective on the ethical considerations associated with snowball sampling and provides guidance for researchers, evaluators, and market researchers seeking to use this methodology ethically and effectively.

The ethical challenges of traditional snowball sampling

Traditional snowball sampling typically involves asking current participants to provide contact details (names, phone numbers, email addresses) of people in their networks who might be eligible and interested in participating. The research/evaluation team then contacts these referred individuals to invite them to participate. This approach is especially useful where there isn't an existing list of people that is readily available, where it is not clear which stakeholders hold relevant information that can answer research/evaluation questions, or where you may want to understand the linkages between stakeholders as well as with the program.

While pragmatic and cost-effective, this approach raises several ethical concerns that warrant careful consideration.

Privacy and unsolicited contact

The most significant ethical issue is that potential participants are unlikely to have consented to being contacted or to having their personal information shared with researchers or evaluators. Chapter 2.3 of the National Statement goes into detail about scenarios where consent to participation may not be possible. While not explicit on the subject of snowball sampling, information sharing by other participants to enable snowball sampling to take place may fall within the scope of such scenarios.

This is because in snowball sampling where contact details are shared without an individual's knowledge:

Their privacy has been compromised without their consent;
They receive unsolicited contact from researchers;
They have had no prior opportunity to decline being approached; and,
Their personal information has been disclosed to third parties without their authorisation.

This is particularly relevant when the research or evaluation involves sensitive topics, populations at higher risk of discomfort or harm, or when potential participants may not have anticipated that their connection to a participant could result in contact from researchers or evaluators.

Autonomy and voluntary participation

Respect for persons as outlined in the National Statement requires that individuals have autonomy over their participation in research. This principle extends beyond the moment of consent to include autonomy over whether one wishes to be approached about research in the first place.

When participants provide contact details of people in their networks, the referred individuals may feel social obligation to participate based on their relationship with the referrer. As a result, voluntary participation may be compromised by relationship-based influence that falls short of overt coercion but nevertheless affects decision-making.

Information asymmetry

In traditional snowball sampling, referred individuals often receive contact from researchers and evaluators with limited context about how their information was obtained. This creates an information asymmetry where:

Potential participants don't know who provided their details;
They may be uncertain about what information was shared about them;
They don't understand the process by which they were identified; and,
This lack of transparency can undermine trust in the research process.

Risk considerations for higher-risk populations

These concerns are heightened when snowball sampling is used to engage with people who may be at a higher risk of discomfort or harm through their participation in research and evaluation. While snowball sampling may be one of few viable methods to reach certain hidden or hard-to-access groups, the very factors that make these populations difficult to reach, such as stigma, marginalisation, privacy concerns, and power imbalances also make privacy breaches and relationship-based pressure more problematic.

Sometimes snowball sampling may be appropriate

Despite these challenges, we recognise that snowball sampling can be an appropriate and even necessary recruitment strategy in certain contexts. Snowball sampling may be justified for:

Hard-to-reach populations: Other recruitment methods are genuinely infeasible for reaching the target population, and the research merit justifies the methodology;
Sensitive communities: The research/evaluation benefits from trust-based referrals within close-knit or marginalised communities where external recruitment would be ineffective or inappropriate;
Resource constraints: The research/evaluation addresses important questions where more privacy-protective methods are genuinely not feasible given legitimate resource limitations; and,
Risk mitigation: The research/evaluation design incorporates appropriate safeguards to minimise privacy intrusion and protect voluntary participation.

However, the mere convenience or cost-effectiveness of snowball sampling does not, on its own, provide sufficient justification for its use where it compromises participant privacy and autonomy. Applicants must demonstrate that the research merit justifies the methodology and that alternative approaches have been genuinely considered.

Our position on ethical snowball sampling

Iris Ethics takes the position that snowball sampling carries inherent risks to participant privacy and autonomy that must be carefully weighed against research merit and mitigated through thoughtful design.

Snowball sampling should be justified

Applications proposing snowball sampling must clearly articulate:

Why this recruitment method is necessary for the research/evaluation objectives;
What alternative recruitment strategies were considered and why they are not viable;
How the research merit justifies the privacy and autonomy implications; and,
What specific population characteristics make snowball sampling the most appropriate approach.

Generic statements about recruitment challenges or resource efficiency are insufficient, and applicants should demonstrate genuine consideration of the ethical trade-offs involved.

Information collection should be minimised

Where snowball sampling is justified, the information solicited from referrers should be limited to the minimum necessary to enable contact. In most cases, this means:

Name and one form of contact details (e.g. email or phone number, not both unless genuinely necessary);
No additional personal information about the referred individual unless directly relevant to eligibility screening; and,
No information about the relationship between the referrer and the referred individual unless this is essential to the research design.

Asking referrers to provide extensive personal information about potential participants is rarely justified and substantially increases privacy intrusion.

Communication materials must be clear and transparent

First contact with referred individuals must include clear, accessible information about:

How their contact details were obtained (e.g., "We received your contact details through our participant referral process");
What information was provided to the research team (e.g., "We were provided with your name and email address only");
Who may have referred them, if this is appropriate to disclose;
That participation is entirely voluntary and there is no obligation to respond;
How to immediately opt out of further contact; and,
How their contact details will be handled if they choose not to participate (i.e., deletion timeframes).

This communication should be carefully worded to be transparent without creating undue concern. The goal is to provide information that enables informed decision-making while maintaining a respectful and professional tone.

Opt-out mechanisms must be immediate and clear

Referred individuals must have simple, immediate mechanisms to decline further contact. This should include:

Clear opt-out instructions in the initial contact (e.g., "If you do not wish to be contacted about this research, please reply to this email with 'remove' or click this link");
Multiple opt-out methods where feasible (e.g., email reply, web link, phone number);
Confirmation that opting out will result in deletion of their contact details; and,
Assurance that opting out will not affect any other relationships or services.

Opt-out processes should not require extensive explanation or justification. A simple expression of non-interest should be sufficient to immediately cease contact and trigger data deletion.

Consider alternative opt-in approaches

Where feasible, we strongly encourage applicants to consider modified opt-in snowball approaches that better protect privacy and autonomy:

Referral card or information-sharing approach:

Current participants are given information materials (cards, flyers, web links) about the research;
Participants share these materials with people in their networks who might be interested;
Interested individuals then contact the research/evaluation team themselves; and,
No personal information is shared without the referred person's knowledge and consent.

Facilitated introduction approach:

Current participants ask people in their networks if they would be interested in hearing about the research/evaluation;
Only if the individual expresses interest does the participant share their contact details with the research/evaluation team; and,
The research/evaluation team then makes contact, noting that the person indicated interest through the referral process.

These approaches maintain the network-based benefits of snowball sampling while significantly reducing privacy intrusion and relationship-based pressure. While they may result in lower recruitment numbers, they better align with the ethical principles of respect and autonomy.

Snowball sampling and ethical review requirements

Snowball sampling does not meet the criteria for exemption from ethical review under the National Statement. While paragraph 5.1.22 provides for negligible risk research to be exempted from HREC review, snowball sampling carries risks to privacy and autonomy that require ethical oversight.

Applications proposing snowball sampling should:

Include snowball sampling in the scope of ethical review: The recruitment methodology should be clearly described in the ethics application, including how referrals will be solicited, what information will be collected, and how referred individuals will be contacted;
Submit communication materials for review: All materials that will be used in the referral process and first contact should be included in the application for committee review. This includes scripts for soliciting referrals, templates for initial contact with referred individuals, and opt-out procedures;
Be submitted for full HREC or lower risk review as appropriate: Depending on the overall project risk profile, snowball sampling may be reviewed through either a full HREC process or our lower risk review process. However, it does not qualify for exemption; and,
Address snowball-specific risks in the risk assessment: The application should explicitly address how privacy risks and potential coercion through social relationships will be mitigated.

Where snowball sampling is one component of a multi-method recruitment strategy, all recruitment approaches should be included in the scope of review to enable the committee to assess the overall ethical approach to participant recruitment.

Special considerations for higher risk individuals

When snowball sampling is proposed for cases where participants may be at a higher risk of discomfort or harm due to their circumstances, additional safeguards should be in place:

Enhanced privacy protection: Consider whether referred individuals' connection to the research topic could itself pose risks (e.g., revealing membership in a stigmatised group);
Power dynamics: Assess whether social hierarchies within the community could lead to pressure to participate or share contact details;
Cultural appropriateness: Ensure that the referral approach is culturally appropriate and that communication materials are accessible and respectful;
Community consultation: Where feasible, involve community representatives in designing the recruitment approach to ensure it aligns with community norms and values; and,
Additional informed consent considerations: Provide extra attention to ensuring that consent is truly voluntary and that participants understand they can withdraw without consequences.

For research involving Aboriginal and Torres Strait Islander peoples, researchers should refer to Chapter 4.7 of the National Statement and the AIATSIS Code of Ethics and ensure that proposed snowball sampling approaches have been discussed with and endorsed by relevant communities, advisory groups, or community-controlled organisations.

Practical guidance for applicants

Based on our review experience, we offer the following practical guidance for social researchers, evaluators, and market researchers considering snowball sampling:

Before deciding on snowball sampling

Genuinely explore alternative recruitment methods and document this consideration;
Assess whether the convenience of snowball sampling justifies the privacy and autonomy implications;
Consider whether opt-in referral approaches (information-sharing rather than contact-detail-sharing) could achieve your objectives; and,
If working with populations where risks may be higher, consult with community representatives or relevant organisations about appropriate recruitment approaches.

If snowball sampling is justified

Minimise the information collected from referrers to name and one form of contact details;
Develop clear, transparent communication materials that explain how contact details were obtained;
Create simple, immediate opt-out mechanisms and commit to prompt data deletion for those who decline;
Consider time-limited storage of referred contact details (e.g., automatic deletion after 30 days if no response);
Train research/evaluation teams on how to respond to questions or concerns from referred individuals;
Document the referral process carefully, including how many referrals were made, response rates, and opt-out rates; and,
Be prepared to modify your approach if opt-out rates or participant feedback suggest the methodology is problematic.

In your ethics application

Clearly justify why snowball sampling is necessary and appropriate;
Describe exactly what information will be collected from referrers;
Include all communication materials for review (referral solicitation, initial contact, opt-out procedures);
Explain how you will maintain records of referrals and opt-outs;
Address privacy and coercion risks explicitly in your risk assessment; and,
If using modified opt-in approaches, describe these clearly so the committee understands how your approach differs from traditional snowball sampling.

Summary

Snowball sampling is a valuable recruitment methodology that, when carefully designed and implemented, can enable important research and evaluation with hard-to-reach populations. However, it carries inherent risks to privacy and autonomy that must not be overlooked in the interest of convenience or efficiency.

Our position is that:

Snowball sampling carries risks to participant privacy and autonomy that must be carefully weighed against research merit;
Use should be justified: Applicants must demonstrate why snowball sampling is necessary and appropriate, not merely convenient;
Information collection should be minimised to what is strictly necessary to enable contact;
Communication materials must be clear and transparent about how contact details were obtained, what information was shared, and how to opt out;
Opt-in referral approaches should be preferred where feasible, as they better protect privacy and autonomy;
Snowball sampling requires ethical review and does not meet the criteria for exemption from review. Applications should include clear description of the referral process and all communication materials; and,
Special care is required for higher-risk populations, including enhanced privacy protection and attention to power dynamics and cultural appropriateness.

By approaching snowball sampling with careful attention to these ethical considerations, researchers can maintain the methodological benefits of network-based recruitment while upholding the core principles of the National Statement: respect for persons, beneficence, justice, and research merit and integrity.

Iris Ethics is committed to working with applicants to ensure that snowball sampling is implemented ethically and appropriately. We encourage applicants to contact us during the application development process if they have questions about how to design snowball sampling approaches that balance recruitment effectiveness with ethical rigour.

Ultimately, ethical research and evaluation is about more than regulatory compliance. It is about demonstrating respect for the people who contribute to our knowledge and ensuring that the pursuit of understanding does not come at the cost of dignity, privacy, or autonomy.

AI Disclosure: Initial drafts of the content for this article were prepared using Large Language Models with input from Iris Ethics staff who guided the scope and design. Subsequent revisions and final versions were developed and approved by Iris Ethics staff.

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