Statistics - Module 1 Flashcards
Def of population
The whole set of items that are of interest
Def of census
Observes or measures every member of a population
Sample def
A selection of observations taken from a subset of the population which is used to find out information about the population as a whole
Advantages of a census
Should give a completely accurate result
Disadvantages of a census
Time consuming and expensive
Cannot be used when the testing process destroys the item
Hard to process large quantity of data
Advantages of samples
Less time consuming and less expensive than a census
Fewer people have to respond
Less data to process than in a census
Disadvantages of sampling
The data may not be as accurate
The sample may not be large enough to give information about small sub-groups of the population
What are sampling units
Individual units of a population
What is a sampling frame?
Sampling units of a population that are individually named or numbered to form a list
What are the three methods of random sampling?
Simple random sampling
Systematic sampling
Stratified sampling
What is simple random sampling?
When the sample of size ‘n’ is one where every sample of size ‘n’ has an equal chance of being selected
Process for random sampling
Acquire a sample frame - usually a list of people or things
Allocate each person or thing a unique number
Choose a number at random either by a random number generator or lottery sampling
What is lottery sampling?
The members of the sampling frame could be written on tickets and placed into a ‘hat’.
The required number of tickets would then be drawn out.
What is systematic sampling?
When the required elements are chosen at regular intervals from an ordered list.
E.g. for a sample of 20 from population 100,
You would take every fifth person, since 100/20 = 5
The first person to be chosen should be chosen at random, e.g. 2, 7, 12, etc.
What is stratified sampling?
The population is divided into mutually exclusive strata(e.g. males and females) and a random sample is taken from each.
Process of stratified sampling
Proportion of each strata sampled should be the same.
You can use a formula to calculate the number of people you should sample from each stratum:
No. in stratum/no. in population x overall sample size
Advantages of simple random sampling
Free of bias
Easy and cheap to implement for small populations and small samples
Each sampling unit has a known and equal chance of selection
Disadvantages of simple random sampling
Not suitable when the population size or the sample size is large
A sampling frame is needed
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
It can introduce bias if the sampling frame is not random
Advantages of stratified sampling
Sample accurately reflects the population structure
Guarantees proportional representation of groups within a population
Disadvantages of stratified sampling
Population must be clearly classified into distinct strata
Selection within each stratum suffers from the same disadvantages as simple random sampling
- need to establish a sampling frame
- if sample size not large enough, can be inaccurate
Two types of non-random sampling
Quota sampling
Opportunity sampling
What is quota sampling?
Where an interviewer or researcher selects a sample that reflects the characteristics of the whole population
