Answer: Snowball sampling
Explanation: Snowball sampling also called chain referral sampling occurs when a research participants request the assistance of other participants for a test or study. It is used where potential participants are hard to find. In snowball sampling a researcher picks the first few samples and either recruits them or asks them to recommend other subjects they know who fit the description of samples needed. This referral technique goes on and on, increasing the size of the respondent population like a snowball rolling down a hill until the researcher has sufficient data to analyze. It's called snowball sampling because once you have the ball rolling, it picks up more “snow” along the way and becomes larger and larger. As evident in the above question Kerry Sobiski employed the assistant of students to recommend other students they have drank with in the past two month, this assistant cascaded further until sufficient enough data was gardered. Without doubt this sampling method is the snowball sampling method.