Population & Sample Flashcards
What is the target population?
Population which we aim to apply our results to
What is the source population?
Sampling frame - list of accessible members of target population
What is the eligible population?
Large fraction of the source population will be excluded when study criteria are applied i.e. ineligible
Who are study entrants?
When eligible population are approached, large portion will be excluded due to e.g. lack of consent, non-response etc.
Those left are the study entrants
Who are the study participants?
Not all the study entrants will complete study requirements or be available for follow-up. The remnants are in this group.
What is sampling error?
Difference between outcome value estimated from sample and true-whole population outcome one might get by including everyone in the population
What does sampling error depend on?
Sampling technique
Sample size
What type of sampling reduces sampling error?
Random sampling
What sample sizes produce less sampling error?
Large sample sizes
What is non-probability sampling
Non-random sampling
Disadvantages of non-random sampling
Generalisability is weaker
Types of non-random sampling
Quota Convenience Volunteer Snowball Consecutive Purposive
What is quota sampling?
Sampling frame divided into subgroups called strata based on selected variables
Non-random sample picked from each stratum
What is snowball sampling?
One identified subject helps to locate another subject
What is consecutive sampling?
Case series of consecutive patients with conditions of interest
What happens in consecutive sampling
All patients who present to research site at time of study are included
What types of studies are purposive sampling used in?
Qualitative
What happens in purposive sampling?
Subjects picked deliberately based on pre-requisite objectives
Types of purposive sampling
Extreme
Critical case
Theoretical purposive
What happens in extreme sampling?
Cases representing two extremes are recruited
What happens in critical case sampling?
Subjects expected to yield maximum information are selected
What happens in theoretical purposive sampling?
Subsequent recruitment depends on those categories that emerged from data so far
Another name for systematic sampling
Interval sampling
What happens in interval sampling?
Every nth item from sampling frame is selected
Difference between random and non-random sampling
In random sampling, each individual in the population has an equal chance of being chosen
