STATISTICS - Data collection Flashcards
Definition of a population
The whole set of items that are of interest
Definition of a sample
Some subset of the population intended to represent the population
Definition of a sampling unit
Each individual thing in the population that can be sampled
Definition of a sampling frame
A list of all the sampling units
Definition of a census
Data collected from the entire population
Advantages of a census
- Should give completely accurate results
- More reliable
Disadvantages of a census
- More time consuming
- More expensive
- Cannot be used when testing involves destruction
- Large volume of data to process
Advantages of a sample
- Cheaper
- Quicker
- Less data to process
- Possible when data involves destruction
Disadvantages of a sample
- Data may not be accurate
- Data may not be large enough to represent small sub-groups
What is qualitative data?
Non - numerical values
What is quantitative data?
Numerical values
What is discrete data?
Can only take specific values
What is continuous data?
Can take any decimal values
What are the types of sampling?
Random vs Non-random sampling
What are the 3 types of random sampling?
- Simple random sample
- Systematic sampling
- Stratified sampling
What are the 2 types of non-random sampling?
- Quota sampling
- Opportunity sampling
How would you carry out a simple random sample?
- List all members of the population and assign them a number from 1-‘n’ to each member
- Use a random number generator to select ‘k’ unique numbers (ignore repeats)
- Select the corresponding members of the population which match the numbers generated to form the sample
Advantages of simple random sampling?
- Bias free
- Easy and cheap to implement
- Each number has a known equal chance of being selected
Disadvantages of simple random sampling?
- Not really suitable when population size is really large
- Sampling frame needed
How would you carry out a systematic sample?
- Take every Kth element where:
> K = population size/sample size - List all the members of the population and assign a number from 1-‘n’ to each
- Use a random number generator to select and number between 1 and K (starting point)
- Then select every Kth person to form the sample
Advantages of systematic sampling?
- Simple and quick to use
- Suitable for large samples/population
Disadvantages of systematic sampling?
- Sampling frame needed
- Can introduce bias if sampling frame is not random
How would you carry out a stratified sample?
For each strata, calculate:
- size of strata/population size x sample size
- Carry out a simple random sample for each strata
Advantages of stratified sampling?
- Reflects population structure
- Guarantees proportional representation of groups within population
Disadvantages of stratified sampling?
- Population must be clearly classified into distinct data
- Selection within each stratum suffers from same disadvantages as simple random sampling
How would you carry out a quota sample?
- Population is divides into groups according to characteristics
- A quota of items/people in each group is set to try and reflect the group’s proportion in the whole population
- Interviewer selects the actual sampling units
Advantages of quota sampling?
- Allows small sample to still be representative of population
- No sampling frame required
- Quick, easy and inexpensive
- Allows for easy comparison between different groups in the population
Disadvantages of quota sampling?
- Non random sampling can introduce bias
- Population must be divided into groups, which can be costly and inaccurate
- Increasing scope of study increases number of groups, adding time/expense
- Non-responses are not recorded
How would you carry out an opportunity sample?
Sample taken from people who are available at time of study, who meet criteria
Advantages of opportunity sampling?
- Easy to carry out
- Inexpensive
Disadvantages of opportunity sampling?
- Unlikely to provide a representative sample
- Highly dependent on individual researcher
Give an example of when a census would be practical and useful (PPQ)
A small company wishes to ask about their employee’s opinions about pension schemes
Give an example of when a census would be impractical and therefore a sample would be needed (PPQ)
A city council wants to find out what its residents feel about their recycling centres
Give an example of how you could use simple random sampling to select a sample of 50 people from an alphabetical list of the 2500 inhabitants of a particular district (PPQ)
- Allocate each person a number starting at 1 and ending at 2500
- Use random number tables to select 50 random numbers between 1 and 2500
