Sampling for Research

Types of Research

• Quantitative– the collection & analysis of data to describe,

explain, predict, or control phenomena of interest• Qualitative– the collection, analysis, & interpretation of

comprehensive narrative and visual data to gain insights into particular phenomenon of interest

• Mixed-Methods

Samples for Quantitative Studies

• Generalizability– Sample chosen should be representative of the

population being examined• Sample Size– The larger the population, the smaller %age

required to get a representative sample• 50% for a sample of 500, 20% for a sample of 1,500

– For smaller populations (n=100 or fewer), the entire population is typically sampled.

Samples for Quantitative Studies

The selection process itself is critical. For example, a voter survey that systematically excluded a particular race, class or gender would be worthless...and there are a host of other ways to ruin, or bias, a sample.

Samples for Quantitative Studies

• Sample Error– A random, unknown characteristic of the sample– causes the sample to not be as representative as

1st thought• Sample Bias– A systematic sampling error– Typically caused by the researcher

Samples for Quantitative Studies

Methods of Sampling:• Simple Random Sample - every member of the

population has an equal chance of being selected

Samples for Quantitative StudiesMethods of Sampling:

• Stratified Sample - Population is divided into groups. Then a random sample is taken from each group

Samples for Quantitative Studies

Methods of Sampling:• Cluster Sample - Population is divided into groups.

Then an entire group is randomly chosen with all members of the randomly chosen group used in the sample

Non-Random Sampling MethodsNon-Random Methods of Sampling:

• Convenience Sample - A sample of well...convenience. Not scientific, just easy. Conclusions about the population cannot be drawn from this type of sample.

• Purposive Sample • Judgment sampling• The process of selecting a sample that is believed

to be representative of a given population. • Quota Sample – Process of selecting a sample

based on required, exact numbers (quotas) of individuals or groups with varying characteristics.