(The following effort at statistical humor comes from S. Huck)
BACKGROUND AND BUBBA’S QUESTION:
True to the weather forecast, the college campus was being blasted by a heavy snowfall. Inside a small dining hall, students were eating, studying, talking, & texting. Suddenly, Bubba darted outside where he scooped up some of the white stuff, packed it together in his hands, and then quickly returned inside. Getting everyone’s attention, Bubba held up the cold, white sphere he had just made and said: “Hey, I learned about this in my stats course. Guess what it is?”
BUBBA’S ANSWER: “A snowball sample!”
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Beyond the Joke: Things worth knowing about snowball samples:
1. Definition: A snowball sample is formed during the time period when people are being recruited to serve as a study’s research participants. Through face-to-face contact or indirect methods (such as posted notices), the researcher successfully solicits certain individuals to voluntarily enter the study. Next, those initial volunteers are asked to recruit additional participants. This process—of existing volunteers recruiting new volunteers—continues until the desired sample size has been achieved.
2. Idea Behind the Name: Imagine a snowball rolling down a steep, snow-covered hill. At first, the snowball is small. But it gets larger and larger as it heads toward to the bottom of the hill. In a similar fashion, a snowball sample grows in size as volunteer participants successfully recruit additional participants.
3. When Used: Snowball samples are used mainly in studies wherein (1) the researcher doesn’t know who the potential participants are or how to contact them, or (2) potential volunteers are more likely to agree to be in a study if they are recruited by a “peer” rather than by an unknown researcher.
4. Example: In a research report entitled “The ‘Staying Safe’ Intervention: Training People Who Inject Drugs in Strategies to Avoid Injection-Related HCV and HIV Infections” (from the journal: AIDS EDUCATION AND PREVENTION), the researchers stated that “Snowball sampling of participants began with eight participants directly recruited from two sources…. These eight participants then recruited 60 eligible peers.”
5. Quality: Because of the way snowball samples are formed, it is difficult to generalize information about them to larger populations. (Such generalizations are much easier to make with stratified random samples and other kinds of samples classified as “probability samples.”) Thus, snowball samples are most useful in studies wherein (1) the goal is to generate rather than confirm hypotheses or (2) the participants, collectively, are considered to be the target group of interest.