Sampling Techniques Flashcards
What is a population?
The whole set of items of interest
What is a census?
An observation or measurement of every member of a population
What is a sample?
A selection of observations taken from a subset of the population, representing the population as a whole
Advantages and disadvantages of a census?
- Gives a completely accurate result
However: - Time consuming and expensive
- Cannot be used when the testing destroys the item
- Hard to process large quantities of data
Advantages and disadvantages of a sample?
- Less time consuming and expensive
- Less people have to respond
- Less data to process
However: - Data may not be as accurate
- Sample may not be large enough to give information about sub groups of the population
What is a sampling unit?
An individual member of the population
What is a sampling frame?
An ordered list of all the sampling units
What is simple random sampling?
Each sample unit has an equal chance of being selected using a method such as a hat or random number generator
What is systematic sampling?
The required elements are chosen at intervals from an ordered list (sampling frame)
What is stratified sampling?
The population is divided into mutually exclusive strata, and a random sample is taken from each
Advantages and disadvantages of a simple random sample?
- Free of bias
- Easy and cheap for small populations and small samples
- Each sampling unit has an equal chance of selection
However: - Can be time consuming, disruptive and expensive for a large sample size
- A sampling frame is needed
Advantages and disadvantages of a systematic sample?
- Simple and quick
- Suitable for large samples and populations
However: - Can introduce bias if the sampling frame is not random
- A sampling frame is needed
Advantages and disadvantages of a stratified sample?
- Accurately reflects population structure
- Guarantees proportional representation of groups
However: - Population must be clearly classified into strata
- Selection within each stratum suffers from the disadvantages simple random sampling
What is quota sampling?
An interviewer or researcher selects a sample that reflects the characteristics of the whole population, and each person is observed
What is opportunity sampling?
A sample is taken from the people who are available at the time of the study and who fit the criteria you are looking for
Advantages and disadvantages of a quota sample?
- Allows a small sample to be representative of the population
- No sampling frame required
- Quick, easy and inexpensive
- Allows for easy comparison between groups
However: - Non random sampling can introduce bias
- Population must be divided into groups, which can be costly or inaccurate
- Increasing the scope of the study increases groups, therefore time and expense
- Non responses are not recorded as such
Advantages and disadvantages of an opportunity sample?
- Easy to carry out
- Inexpensive
However: - Unlikely to provide a representative sample
- Highly dependent of the individual researcher
How do you calculate the number of samples to take per stratum in a stratified sample?
(Number in stratum/number in population) * overall sample size
