Chapter 1 Data Collection (AS) Flashcards
What is a population?
A population consists of all the items that are of interest
What is a sample?
A sample is a subset of items chosen from a population
What is a sampling unit?
Each individual item in the population that can be sampled
What is the sampling frame?
An ordered (numbered) list of sampling units
What is a census?
data collected from the entire population
What are the advantages of a census?
It should give completely accurate results
What are the advantages of a sample?
- Less expensive
- Less time consuming
- Less data to process
What are the disadvantages of a census?
- Time consuming and expensive
- Cannot be used when testing involves destruction
- Large volumes of data to process
What are the disadvantages of a sample?
- Data may not be accurate
- Sample may not be large enough to represent small sub-groups of the population
What is random sampling?
When each member of the population has an equal chance of being chosen in order to avoid bias
What are the 3 types of random sampling?
- Simple Random
- Systematic
- Stratified
What is the method for a simple random sample?
To carry out a simple random sample, size n, from a population size N, firstly you need a sampling frame.
Each item is assigned a different number from 1-N. Use a random number generator to select n unique numbers
Choose the items corresponding to these numbers to form the sample
What is the method for a systematic sample?
To carry out a systematic sample, size n, from a population size N, you need a samplin frame.
Each item is assigned a random number from 1-N. K = N/n. Use a random number generator to select a random number between 1 and N, starting with this number, take every Kth element to form the sample.
What is the method for a stratified sample?
To carry out a stratified sample, size n, from population size N, you need a sampling frame and distinct strata. The same proportion (n/N) is to be sampled from each strata
Within each strata, each item is assigend a different number and a random number generator is used to select the number of unique numbers required.
Choose the items corresponding to these numbers to form the sample
What is a systematic sample?
The required elements are chosen at regular intervals from an ordered list
What is a stratified sample?
Population is divided into groups (strata) and a simple random sample is carried out in each group
What are the advantages of simple random sampling?
- Easy and cheap to implement for small populations and samples
- Each sampling unit has an equal chance of being seleceted and it is therefore bias free
What are the disadvantages of simple random sampling?
- Not suitable when population size is large
- Sample may not accurately reflect the population
- A sampling frame is needed
What are the advantages of systematic sampling?
- Simple and quick to use
- Suitable for large samples and populations
What are the disadvantages of systematic sampling?
- Can introduce bias if sampling frame is small and not random as patterns can be picked up in the data
- A sampling frame is needed
What are the advantages of stratified sampling?
- Sample accurately reflects population structure
- Guarantees proportional representation of groups within population
What are the disadvantages of stratified sampling?
- Sampling frame is needed and population must be clearly classified into distinct strata
- Selection within each stratum suffers from same disadvantages as simple random sampling
What are the 2 types of non-random sampling?
- Quota Sampling
- Opportunity/Conveninnce Sampling
What is quota sampling?
- The population is divided into groups according to characteristic.
- A quota of items in each group is set to try and reflect the group’s proportion in the whole population (quoatas calculated in the same way as stratified sampling).
- Interviewer selects the actual sampling units until the quotas are reached.
- Once a quota is full, ignore any subsequent sampling units that also meet the characteristic
What is opportunity/convenience sampling?
Sample taken from people who are available at time of study, who meet criteria
What are the advantages of quota sampling?
- Allows a small sample to still be representative of population
- No sampling frame required
- Relatively quick, easy, and inexpensive
What are the disadvantages of quota sampling?
- Non-random sampling can introduce bias
- Population must be divided into groups, which can be costly or inaccurate
- Can depend on knowledge/expertise of researchers
What are the advantages of opportunity/convenience sampling?
- Easy to carry out
- Inexpensive
What are the disadvantages of opportunity/convenience sampling?
- Unlikely to provide a representative sample
- Highly dependent on individual researcher
What is the difference between quota sampling and stratified sampling?
In stratified sampling, a simple random sample is carried out in each subgroup, whereas in quota sampling, an opportunity sample is carried out in each subgroup
What is qualitative/categorical data?
non-numerical values
What is quantitative data?
numerical values
What are the 2 types of quantitative data?
Discrete and Continuous
What is Discrete Data?
Can only take place at specific values (counted data)
What is continuous data?
Can take any decimal value (measured data)
What are the advantages/disadvantages of grouping data?
- Creates conciseness
- Loss of exact original values
