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Guidelines for Investigators Using Snowball Sampling Recruitment Methods

Snowball sampling is a recruitment technique in which research participants are asked to assist researchers in identifying other potential subjects.  The use of currently enrolled research participants to recruit additional research participants (sometimes referred to as “the snowball sampling”) may be approved by the IRB under some circumstances.  However, the protocol must include justification of the use of this method in the context of the study and target population.  The method that minimizes risk would be the preferred choice.  For example, a researcher seeking to study patterns of informal leadership in a community may ask individuals to name others who are influential in a community.

This recruitment procedure is sometimes inappropriately labeled “cold calling.”  The following excerpt from the Belmont Report regarding voluntariness compels IRBs to pay close attention to snowball recruitment procedures:

Unjustifiable pressures usually occur when persons in positions of authority or commanding influence—especially where possible sanctions are involved— urge a course of action for a subject. A continuum of such influencing factors exists, however, and it is impossible to state precisely where justifiable persuasion ends and undue influence begins.

For example, what if a highly respected individual in the community (say a local elected official, teacher, member of the clergy, or tribal elder) provided the name of a potential contact?  When the nominee is contacted, the person might be unduly influenced to participate in the project because of the stature of the nominator. Consideration of minimal risk is likely to play an important role in IRB review of snowball recruitment procedures. For example, it would make a big difference whether the research involved extramarital affairs or substance use as opposed to family meal time routines or leisure activities. To “cold call” someone on sensitive topic issues places him or her in a potentially embarrassing situation, which should be avoided. Moreover, the original subject is being placed in the position of being asked to disclose information about a third party without that party’s consent.

Snowball recruitment where the researcher obtains names and contact information from one individual for another one, particularly for research involving sensitive topics, is generally not considered appropriate by the Boise State IRB. Using such a method could result in the researcher obtaining information about an individual that is sensitive and private without their permission. Alternatively, the researcher should ask other subjects to pass along flyers, emails, information sheets, etc. that contain contact information for the researcher to prevent a breach in confidentiality or an invasion of privacy. In instances where the research subject is not sensitive (i.e., opinions on politics, consumer opinions), a snowball recruitment method where one person provides the name and contact information may be acceptable. If the topic is sensitive or personal, snowball sampling may be justified, but care should be taken to ensure that the potential subjects’ privacy is not violated.   For example, studies of networks of drug users or studies tracking sex partners require extreme caution with information gathered from one subject about another.

The steps taken to minimize the risk of violating an individual’s privacy should be articulated in the recruitment section of the protocol.  Current participants cannot receive incentives or compensation for referrals.

One means of reducing the risks in snowball recruitment is for the researcher to gently ask subjects whether they would be willing to pass some information about the study they just completed to other potential subjects. The researcher may give the original subject an information sheet or flyer that they can give to people they think might be interested and qualified. Then these potential participants can contact the researcher if they are interested. The researcher must provide the IRB with a copy of the recruitment item (which must include all of the same information that is in the initial recruitment script) he/she will give to the initial contact to distribute. If the potential subject were interested in study participation, they would be free to contact the researcher.

Some Reasons Snowball Recruitment Could Be Approved

Snowball recruitment could be approved for minimal risk projects for reasons such as:

  • People cannot always be depended upon to make phone calls and contacts. Even with good intentions, they forget, become busy, etc.;
  • If participants have to contact others, they may have to use their time to make at least two phone calls (one to other potential subjects and one to the investigator). Participants shouldn’t be required to do extra work if there is no practical risk involved;
  • Participants might not accurately describe the research and its purposes or they might prep other participants to provide particular responses.


The IRB recommends the following safeguard procedures if snowball recruitment is to be used:

  • Let subjects know that they will be asked to provide the names of other potential recruits, but that they have the right to decline to provide this information.
  • Obtain the nominating subject’s written permission to reveal their identity to subsequent contacts on the consent form via an initialed check list item. That is, if the subject agrees to provide names of other possible contacts, they could initial a check box on the consent form that they agree to allow their identity to be revealed to the subsequent contacts. Not initialing the box would be interpreted as meaning the subject was unwilling to permit his/her name to be used when making the additional contacts, even if he/she supplied the names and contact information.
  • Provide the IRB with a script detailing how the researcher will be obtaining the information for the snowball recruitment.

Acceptable alternatives that reach the same potential subjects include:

  • The study team member may provide information to subjects and encourage them to pass it on to others who may be interested or eligible. The information provided to enrolled subjects (fliers, letters of explanation, etc.) must be approved by the IRB. Interested prospective participants could then contact the project for more info and possible inclusion.
  • The study team member may ask subjects to obtain permission from others prior to disclosing their contact information.  In this scenario, the researcher would not directly contact the referred/potential subject without permission from the potential subject and would not have access to any information about a potential subject without permission from that individual.

Examples provided by the National Science Foundation.

Sample Snowball Recruitment Letter

Dear [Mr. / Ms. LAST NAME],

Thank you for your interest in [NAME OF STUDY].  I am writing to ask whether you would be willing to pass along the enclosed information to friends and/or family members who may also be interested in learning about this research study.  You are under no obligation to share this information and whether or not you share this information will not affect your relationship with the staff at Boise State University.

Thank you for your time and consideration.


[PI signature and name]

Include enclosure(s) as applicable:

Recruitment materials, e.g. brochure, flyers, consent form, etc.

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