How to Choose a Sample for Your Thesis

Before talking about sampling, you need to have some concepts clear

Concepts to KnowResearch Population The whole set of units (people, groups,

events, settings, etc.) on which the research is focused and the findings are expected to be generalized.

Sample Representative subset of the population the researcher studies when the whole population is not available

Sampling Scheme Specific strategies to select samplesSampling Design Umbrella term that includes both

selecting strategies and sample size

Relationships to Remember

Research Problem

• What needs to be studied under a specific situation

Research Question

• What to answer or test

Methodology • How to gather and

analyze valid data

Elements to Gather Valid Information

Valid Information to Answer or Test Research Question

Data Analysis Methods

Samplin

g Design

Research

Design

Next slides will present a basic summary of some of the sampling schemes that are widely used in research.

However, be aware that the application of probabilistic (random) schemes to quantitative research and non-probabilistic (non-random) ones to qualitative studies does not imply that these are the only approaches available.

Onwuegbuzie, A. & Collins, K. (2007). A typology of mixed methods sampling designs in social science research. The Qualitative Report, 12 (2), 281-316

Some form of non-random scheme is the most common choice used in both quantitative and qualitative studies and random ones for quantitative research and non-random for qualitative studies is the second most common combination (Onwuegbuzie & Collins, 2007).

Such predominance of non-random samples seems to be related to the fact that most studies in social sciences cannot be done under experimental conditions in which pure random selection is expected.

Quantitative ResearchSimple random The whole population is available and any

unit has the same chance of being chosen Stratified random The whole population is divided into

subpopulations (strata) with respect to one or more characteristics that interest and units are selected from each stratum at random. Allocation of units can be done equal or proportional to the population

Systematic Population is ordered according to a criterion and units are chosen from the list by selecting every nth one

Cluster Population consists of limited groups (clusters) and sampling is focused on selecting clusters instead of individual units. Not all clusters are included in the sample

Multistage random Very large populations are divided into clusters and then sub-clusters and units are selected at random following a general to specific direction

Qualitative ResearchConvenience Sample consists of units that are available and/ or

willing to participate

Purposeful Researcher is interested in studying specific groups. Selection of units can be done at random, stratified, or using more than one scheme (mixed)

Quota Units are selected in relation to pre-defined characteristics either in proportion to population sub-groups or minimum number from each sub-group

Snowball Similar units are required and access to them is done by asking participants to recommend peers

Multistage purposeful Units are selected in more than one stage and always applying a purposive scheme

Qualitative ResearchTypical case Units are chosen because they represent the average

element of what is studied

Maximum variation When differences are the research target, dissimilar units are chosen to form sample

Criterion Units are selected because each one of them represents one or more desired criteria

Theoretical Units are chosen because they can provide input information to build or test a theory

Mixed Methods ResearchAll previous schemes When choosing samples for mixed methods studies,

researchers need to take into account the purpose of the study, the research questions, and the stages in which the study will be developed.

All those elements help them select the most appropriate sampling scheme for each stage. When generalization is the main concern of a stage, the first five schemes presented are the best options. When understanding is the focus of a stage, then the rest of schemes should be considered.

Cases to Practice Sampling

Quantitative ResearchThe research director of an educational city system, that includes forty-five elementary schools, wants to answer the following research question:

What’s the achievement in mathematics of fifth-grade students of the school system, measured by a standardized achievement test?

Options for Sampling:

1. Simple random2. Stratified random3. Cluster

And the answer is…….

Cluster SamplingIt is too expensive to administer the test to the whole population and the same happens if the simple random sampling is chosen.

Stratified random sampling implies administering the test to some of the students of a class and not to others and that may be inconvenient from the students’ point of view.

Since students are organized in classes –which are in fact clusters, the most appropriate option is cluster sampling and that will allow to test all students in the chosen classes.

Qualitative Research

A researcher wants to know the reasons why some employees of large companies have strong pro-mentoring duties views.

Options for Sampling:

1. Purposeful2. Snowball3. Maximum variation

And the answer is…….

Snowball

Since participants with similar views within organizations are required and they could be difficult to locate, it is appropriate to ask them to recommend potential subjects.

Sample Size

Criteria to Select Sample Size• Costs (money, time, and effort) to get sample data.

• For quantitative studies: • Population size (the larger the population, the larger

the sample)• Confidence interval (how much error will be allowed)• Confidence level (how much confident you want to

be that your results are within the selected confidence interval)

• Standard deviation (how much dispersion from the mean you expect)

These criteria are used by calculators to determine the most appropriate sample size for your study.

Criteria to Select Sample Size

For qualitative studies: Samples are usually small, but the precise number is very unlikely to be determined at the beginning of the study .

The main criterion is to have a sample as big as needed to have all the information that might be important. Therefore, when information becomes redundant, the sample size has been reached.

Criteria to Select Sample Size

For mixed studies: • Usually the sizes of the sample for each stage

are different

• The mixed methods variant used influences the sample sizes. For example, in an explanatory design the same participants must be included in all the stages while in an exploratory design more participants are needed in the quantitative stage.

Advanced Idea for Today:

More than one criterion is needed to select the best sampling design to gather valid data.

Now You Know

1. The relationship between research problem, research question, and sampling.

2. Some of the basic sampling schemes you may use to select participants for your study