6.2 Sampling

Question 1

Sampling

Answer 1

• Process of selecting representative units of a population in a study
• Usually found in the “methods” section

Question 2

Definitions

Answer 2

Population
- The entire group of elements that meet the study’s inclusion requirements

Accessible Population
- Population that meets the target population criteria and is available

Sample
- Set of elements that make up the population

Inclusion Criteria
- Characteristics that restrict the population to a homogenous group of subjects (Eligible Criteria)

Exclusion Criteria
- Characteristics that restrict the population to a homogenous group of subjects

Question 3

Samples/Sampling

Answer 3

• Selection of a portion of the population to represent the entire population
• When done properly claims can be made about the population based on data from the sample alone.
• Sample size and method of selection determines how well data is represented in the population

Question 4

Nonprobability Sampling

Answer 4

• Elements are chosen with non-random methods

DRAWBACK
- Findings are less generalizable
- No way of estimating the probability of an element being included in a particular sample

• Carefully choose inclusion and exclusion criteria and have an optimal sample size to increase sample representativeness of the population

Types
- Convenience Sampling
- Quota Sampling
- Purposive Sampling
- Network (Snowball) Sampling

Question 5

Probability Sampling

Answer 5

• Random Sampling (Selection)
• Goal is to obtain representative elements of populations
• Random means each element of the population has an equal and independent chance of being included in the sample.
• Element (members of the population) are selected from a sampling frame (lists all elements of a population)

ADVANTAGE
- Greater confidence that sample is not bias and is representative of the population being studied

TYPES
- Simple random sampling
- Stratified random sampling
- Multistage (Cluster) Sampling

Question 6

Sample Size

Answer 6

Optimal Sample Size Depends On
- Type of design
- Type of sampling procedure
- Type of formula used for estimating optimal sample size
- Degree of precision required
- Heterogeneity of the attributes under investigation
- Frequency of the phenomena of interest occurring in the population
- Projected cost of using a particular sampling strategy

• LARGEST SAMPLE SIZE POSSIBLE IS THE BEST APPROACH
• Samples sizes that are too small are not generalizable
• COHEN’s POWER ANALYSIS TO DETERMINE OPTIMAL SAMPLE SIZE

Question 7

Sample Size

Answer 7

Sometimes “larger sample is better” has exceptions

• Qualitative research tends to be fewer sample size until “data saturation” is reached
• Pilot studies (prelude to larger scale studies or parent studies) also use small sample sizes