BHV 390: Research Methods

Non-Probability Sampling Techniques

Kimberly Porter Martin, Ph.D.

What is a Population?

DEFINITION: The group to which you want to

generalize your findings.

IN OTHER WORDS:The larger group you are representing with your

sample.ORThe larger group to which your results will apply.

What is a Sample?

DEFINITION A subset of the population being

studied from which data is actually collected.

A good sample accurately represents all kinds of elements/members in proportion to their presence in the population.

Sampling Techniques

Sampling techniques are the processes by which the subset of the population from which you will collect data are chosen.

There are TWO general types of sampling techniques:

1) PROBABILITY SAMPLING

2) NON-PROBABILITY SAMPLING

Non-Probability Sampling

DEFINITION

The process of selecting a sample from a population without using (statistical) probability theory.

NOTE THAT IN NON-PROBABILITY SAMPLING• each element/member of the population DOES NOT

have an equal chance of being included in the sample, and

• the researcher CANNOT estimate the error caused by not collecting data from all elements/members of the population.

Types of Non-Probability Sampling

1. Convenient (or Convenience) Sampling

2. Quota Sampling

3. Judgment Sampling

4. Snowball Sampling

Convenient Sampling

DEFINITION

Selecting easily accessible participants with no randomization.

For example, asking people who live in your dorm to take a survey for your project.

Quota Sampling

DEFINITION

Selecting participant in numbers proportionate to their numbers in the larger population, no randomization.

For example you include exactly 50 males and 50 females in a sample of 100.

Judgment Sampling

DEFINITION

Selecting participants because they have certain predetermined characteristics, no randomization.

For example, you want to be sure include African Americans, EuroAmericans, Latinos and Asian Americans in relatively equal numbers.

Snowball Sampling

DEFINITION Selecting participants by finding one

or two participants and then asking them to refer you to others.

For example, meeting a homeless person, interviewing that person, and then asking him/her to introduce you to other homeless people you might interview.

Study Guide

Population

Sample

Probability sampling

Non-probability sampling

Convenient sample

Quota sample

Judgment sample

Snowball sample