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Prepared and presented by Professor Omar Hasan Kasule Sr
DEFINITIONS
- The word population in statistical usage is defined as a set of objects, states, or events with a common observable characteristic or attribute.
- Elements are the members of the population or a sample thereof.
- A sample is a representative subset of the population selected to obtain information on the population.
- A Sampling plan is a whole process of selecting a sample.
- Sampling starts by defining a sampling frame (list of individuals to be sampled).
- The sampling units are the people or objects to be sampled.
- The sample is selected from the study population (population of interest).
RANDOM (PROBABILITY) SAMPLING
- In random sampling, any element has the same inclusion probability.
- Sampling with replacement/without replacement
- Simple random sampling is a random selection from the population used when the population is approximately homogenous.
- Stratified random sampling involves dividing the population into groups called strata and simple random sampling is carried out in each stratum.
- Systematic random sampling is used if an ordered list is available such that every nth unit is included.
- Multi-stage random sampling is simple random sampling of 2 or more stages.
NON-SCIENTIFIC SAMPLING
- Convenience or casual sampling is subjective, depends on whims, and there is no concern about objectivity.
- A quota sample is a subjective selection of a pre-fixed number from each category.
OTHER TYPES OF SAMPLING
- Cluster sampling uses clusters (groups of individuals) as sampling units instead of individuals.
- Epidemiological sampling schemes: cross-sectional, case-control, and follow-up
- Environmental sampling, static or continuous, uses direct measurements and has the advantages of being objective, individualized, quantitative, specific, and sensitive.
SAMPLE SIZE
- Each study design has its specific sample size formula
- Each sample size formula requires an input of information that is inaccurate guessed
- We must understand the limitations of the computed sample