This is a type of purposive sampling used by many qualitative researchers because of its ease of use in identifying a small number of participants. In order to get the general idea, imagine the researcher making a small snowball and then rolling it down a hill; as it goes along, it gets bigger and bigger. The same idea applies to this type of sampling; the researcher identifies a small number of participants, perhaps even one, and then asks those participants to recruit other potential participants for the study they are in.
For example, in the qualitative aspect of the study about hazardous waste being used in the Appendices of this book, researchers could use snowball sampling; study subjects whose health had been negatively affected by living close to the chemical processing plant help identify neighbors and others whose circumstances are similar. Sampling of this type helps the researcher identify potential participants who might otherwise have been missed.
For example, in the qualitative aspect of the study about hazardous waste being used in the Appendices of this book, researchers could use snowball sampling; study subjects whose health had been negatively affected by living close to the chemical processing plant help identify neighbors and others whose circumstances are similar. Sampling of this type helps the researcher identify potential participants who might otherwise have been missed.