Non-probability sampling methods

Module items¶

Learn the basic terms and concepts related to sampling
Identify the differences between probability and non-probability sampling
Compare and contrast
1. Convenience sample,
2. Purposive sample,
3. Snowball sample, and,
4. Quota sample

Population: The universe of units from which the sample is to be selected.
Sample: The segment of population that is selected for investigation.
Sampling: The process of selecting units from a population so that we can generalize our results over the sample.

Main distinction between these sampling types is the random selection of participants.
- In probability sampling a sampling frame is used and participants are selected randomly.
- In non-probability sampling, a sampling frame is not often available, and participants are not selected randomly.
The choice of a sampling strategy depends on the type of study being conducted.
Randomness:
- Everyone in the population has an equal chance of being selected

discussion
- Imagine you are preparing to conduct research involving a sample size of random 250 students from CSUMB.
- Before proceeding, it's essential to verify the effectiveness of your questions in a real-world setting by conducting a pilot study.
- To facilitate this pilot study, you must quickly find 10 students to participate.
- How would you go about achieving this?

The most easily accessible individuals. In this type of sampling, researcher selects any participant who is readily available to participate in the study (e.g., people in your network, people in your classroom).
Their participation happens based on availability.
Useful when piloting a research. This type of sampling can be cost and time effective in the case of pilot studies.

discussion
- Following the pilot study, your advisor recommended concentrating on freshman students and targeting a sample size of 50, instead of the broader group of 250 students initially considered. This advice stems from the outcomes of the pilot study.
- How would you go about achieving this?

Also known as judgmental sampling.
A technique by which the researcher targets the sampling to a very specific population with unique characteristics (e.g., freshman students, high school dropouts, children who were homeschooled).
Homogenous sampling is a type of purposive sampling where participants are chosen based on a trait or characteristic.

discussion
- You’re facing difficulties in finding freshman students, having found only two so far.
- However, after conducting interviews with these two freshmen, you’ve come to understand that they are quite popular among their peers—one leads a student club, while the other has many freshman friends.
- Given this realization that they have extensive connections with other freshmen, what strategies might you consider employing?

Researcher makes initial contact with a small group.
These respondents introduce others in their network.
In this type of sampling participants are selected by word-of-mouth.
Sample may not be representative of the population as participants know each other in some way and may share characteristics such as social class, income, or religion.
Benefits to this strategy include its usefulness in identifying hard to reach populations (e.g., drug traffickers).

discussion
- The two freshmen who have many friends assisted you in identifying 50 freshmen. Meanwhile, you have further explored the literature and discovered that your topic is particularly well-suited for comparing Hispanic and non-Hispanic students.
- Currently, your sample consists of 38 Hispanic and 12 non-Hispanic students, which does not offer a balanced comparative perspective.
- What strategies might you consider employing for such comparison?

A technique that allows the comparison of different groups within a population of interest.
Population
- 50% male, 50% female
- 70% White, 30% Black
Quota Sample
- 50% male, 50% female
- 50% White, 50% Black
Representative of gender distribution in population, not representative of race distribution.