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Sampling and Research Design

The Crux of Data Collection

What is Sampling?

Sampling is the process by which participants are selected from a larger population for research. The choice of sampling method influences the representativeness, validity, and generalisability of research findings.

Core Definitions

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Population

The entire group a researcher is interested in studying.
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Sample

A subset of the population actually studied.
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Sampling Frame

A list or method used to identify members of the population.

1. Probability Sampling

Every member has a known, equal chance of selection; enables generalisation.

A

RANDOM SAMPLING

Participants chosen purely by chance, often using random number generators; reduces bias.
B

SYSTEMATIC SAMPLING

Selecting every nth person from a sampling frame; simpler but still fairly random.
C

STRATIFIED SAMPLING

The population is divided into strata (subgroups) based on characteristics (e.g., gender, age). Samples are randomly selected proportionally from each stratum to ensure representation.
D

CLUSTER SAMPLING

Population divided into clusters (e.g., schools), some clusters randomly selected, and all participants from clusters are studied. Useful for large populations or where sampling frames are difficult.

2. Non-Probability Sampling

Not all members have a known chance of selection; may introduce bias but is easier.

A

OPPORTUNITY (CONVENIENCE) SAMPLING

Selecting participants who are easily available. Common in psychology but risks bias.
B

VOLUNTEER SAMPLING (SELF-SELECTED)

Participants volunteer in response to advertisements. May attract certain types of people.
C

QUOTA SAMPLING

The researcher sets quotas to reflect population characteristics and fills them by convenience sampling.
D

SNOWBALL SAMPLING

Existing participants recruit future participants, used in hard-to-reach populations.

Influencing Factors

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Representativeness: To make valid generalisations, the sample should closely match the population.

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Practicality & Accessibility: Sometimes convenience or time force less ideal methods.

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Research Purpose: Qualitative research may focus on depth rather than representativeness, affecting sampling choices.

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Sample Size: Larger samples tend to be more representative and increase statistical power, but are costlier and time-consuming.

Sampling Bias

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Sampling Bias Defined Occurs when some members of the population are systematically excluded or overrepresented.
  • Examples: Volunteer bias, non-response bias, selection bias.
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Consequences Can limit external validity and generalisability.

Mitigating Issues

✓

Probability Sampling

Use probability sampling where possible.
✓

Describe Procedure

Clearly describe sampling procedure in reports.
✓

Acknowledge Limitations

Acknowledge sampling limitations when interpreting results.
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Sampling in Research Deck
Term
Sampling

What is sampling in research?

Answer
Definition

The process of selecting participants from a larger population for study.

Term
Population

What is a population in research terms?

Answer
Definition

The entire group a researcher is interested in studying.

Term
Sample

What is a sample?

Answer
Definition

A subset of the population actually studied.

Term
Sampling Frame

What is a sampling frame?

Answer
Definition

A list or method used to identify members of the population.

Term
Probability Sampling

What is probability sampling?

Answer
Definition

Sampling where every member has a known, equal chance of selection.

Term
Types of Probability Sampling

Name four types of probability sampling.

Answer
Types

Random, systematic, stratified, cluster sampling.

Term
Non-Probability Sampling

What is non-probability sampling?

Answer
Definition

Sampling where not all members have a known chance of selection.

Term
Types of Non-Probability Sampling

Name four types of non-probability sampling.

Answer
Types

Opportunity (convenience), volunteer, quota, snowball sampling.

Term
Sampling Bias

What is sampling bias?

Answer
Definition

When some population members are systematically excluded or overrepresented.

Term
Representativeness

Why is representativeness important in sampling?

Answer
Importance

It ensures research findings can be validly generalized to the population.

Term
Effects of Sampling Bias

How can sampling bias affect research?

Answer
Effects

Limits external validity and generalisability.

Term
Mitigating Sampling Issues

How can sampling issues be mitigated?

Answer
Mitigation

Use probability sampling, clearly describe procedures, acknowledge limitations.

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📊 Sampling Methods Quiz

1. What is the main advantage of probability sampling?

Probability sampling ensures every member has a known chance, enabling results to apply broadly.

2. Which sampling technique involves dividing the population into subgroups and sampling proportionally?

Stratified sampling uses subgroups (strata) to maintain representation from each group.

3. Which of the following is a non-probability sampling method?

Snowball sampling relies on participants recruiting others and is not random.

4. What does sampling bias affect in research?

Bias distorts who is selected, limiting how well results apply outside the sample.

5. Which factor does NOT influence sampling decisions?

While necessary for sampling, the frame itself is a tool rather than a decision factor.

📊 Results

  • Notes and Quiz
  • Psychology
  • A Level
  • Addiction
  • OCR