Statistics: Sampling Flashcards
What is a Census?
A census measures or observes every member of a population
What is a Sample?
A sample is a selection of observations taken from a subset of a population. It’s used to find out more information about a population as a whole.
Advantages / Disadvantages of a Census
Advantage:
- Results should be completely accurate
Disadvantages:
- Time consuming and expensive
- Hard to process large quantity of data
Advantages / Disadvantages of a Sample
Advantages:
- Less time consuming and cheaper
- Few people have to respond
Disadvantages:
- Data may not be as accurate
- Sample may not be large enough to give info about small subgroups of a population
What is Random sampling?
Each member of a population has an equal chance of being selected.
The three types of Random Sampling:
- Simple random sampling
- Systematic sampling
- Stratified
What is Simple random sampling?
Every sample of size n has an equal chance of being selected.
A Simple random sample can be taken by using:
- A calculator or random number generator
- Lottery sampling
How does a Calculator or Random number generator work?
- Number each member from 1 to n (size of sample)
- Use a calculator / random number generator to generate x random numbers between 1 to n
- Select the members who correspond to the numbers.
How does Lottery Sampling work?
- Write the name of members on identical cards
- Place them in a hat
- Draw up x cards and select these members.
Advantages of Simple random Sampling
- Free of bias
- Easy and cheap for small samples and populations
- Each sampling unit has a known and equal chance of selection
Disadvantages of Simple random Sampling
- Not suitable for large samples and populations
- Sampling frame needed
What is Systematic sampling?
The required elements are chosen at regular intervals from an ORDERED list
Advantages of Systematic sampling
- Simple and quick to use
- Useful for large samples and populations
Disadvantages of Systematic sampling?
- Sampling frame needed
- Bias introduced if sampling frame is not random
What is Stratified sampling?
The population is divided into mutually exclusive strata and a random sample is taken from each.
Num sampled in a stratum = number in stratum
Disadvantages of Systematic sampling
- A sampling frame is needed
- Bias introduced if sampling frame is not random
What is Stratified sampling?
Population is divided into mutually exclusive strata and a random sample is taken from each
Number sampled in statum:
(num in stratum / num in population) x overall sample size
Advantages of Stratified sampling
- Sample accurately reflects population structure
- Proportional representation of group within population
Disadvantages of Stratified sampling
- Population must be clearly classified into distinct strata
- same disadvantages as simple random sampling within each stratum
What is Quota sampling?
An interviewer or researcher selects a sample that reflects the characteristics of the whole population
Advantages of Quota sampling
- No sampling frame required
- Quick, easy and inexpensive
- Allows small sample to still be representative of the population
Disadvantages of Quota sampling
- Non-random sampling can introduce bias
- Population must be divided into groups -> can be costly or inaccurate
What is Opportunity sampling?
Sample is taken from people who are available at the time of study and who fits the criteria you’re looking for
Advantages of Opportunity sampling
- Easy and inexpensive
Disadvantages of Opportunity sampling
- Unlikely to provide a representative result
- Highly dependent on individual researcher
