What is a population in Statistics?
In statistics, population is the whole set of items that are of interest.
What is meant by census in statistics?
A census measures or observes every
member of a population
What is meant by sample in statistics?
A sample is a selection of observations taken from a subset of population and used to find out more information about the population as a whole.
What are the advantages of doing a census?
Results should be completely accurate
What are the disadvantages of doing a census?
- Time consuming and expensive
- Cannot be used when testing destroys process
- Hard to process large quantity of data
What are the advantages of a sample?
- Less time consuming and cheaper
- Fewer people have to respond
- Less data needs to be processed
What are the disadvantages of a sample?
- Data may not be as accurate
- Sample may not be large enough to give information about small subgroups of the population
What are sampling units?
Individual units of a population are known as sampling units
What is a sampling frame?
Sampling units are named and numbered to form a list called a sampling frame
What is meant by Random sampling?
Each member of the population has an equal chance of being selected. The sample should be representative of the population and bias should be removed.
A simple random sample of size π is one where every sample of size π has an equal chance of being selected.
Calculator or random number generator:
Lottery sampling
Advantages of 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 random sampling
- Not suitable for large samples and populations
- Sampling frame needed
What is meant by systematic sampling?
The required elements are chosen at regular intervals from an ordered list.
How are the elements chosen in a systematic sampling? For example a sample of size 20 is needed for a population of 100
Divide the population by the sample size (100/20) = 5
So every 5th person is chosen
The first is chosen at random
Advantages of Systematic sampling
- Simple and quick to use
- Suitable for large samples and large populations
Disadvantages of Systematic sampling
- A sampling frame is needed
- Bias introduced if sampling frame is not random
What is meant by Stratified sampling
The population is divided into mutually exclusive strata and a random sample is taken from each.
How is the Number sampled in a stratum calculated?
Number sampled in a stratum=
number in stratum / number 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 are the two types of non random sampling?
Quota and Opportunity
What is quota sampling?
In quota sampling an interviewer selects a sample that reflects the characteristics of the whole population.
The population is divided into groups according to a given characteristic. The size of each group determines the proportion of the sample that should have that characteristic.
What is opportunity sampling?
Take sample from people who are available at the time the study is carried out and that fit the criteria you are looking for. Eg. the first 20 people you meet outside a supermarket with a carrier bag
Advantages and Disadvantages of Quota Sampling
+
Allows a small sample to still be representative of the population
No sampling frame required
Quick, easy and inexpensive
Allows quick comparisons between different groups in the population
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non random sampling can introduce bias
population must be divided into groups which can be costly or inaccurate
Increasing scope of study increases number of groups which adds time and expenses
non-responses are not recorded as such
