Statistics Chapter 1 Flashcards
What is a Population
The whole set of items of interest
What is a Sample
A subset of the population
What is a sampling unit
Each individual thing in the population that can be sampled
What is a sampling frame
Named/Numbered units of a population to form a list
What is a census?
Where data is collected from the entire population
What are the advantages and disadvantages of a census?
+ Should give a completely accurate result
- Time consuming
- Expensive
- Testing involves destruction
- Large volume of data to test
What are the benefits of a sample compared to a census?
+ Cheaper
+ Quicker
+ Less data to process
- Data may not be accurate
- Data may not be large enough to represent small subgroups
How could you improve a sample?
Use a larger sampling size
What is a simple random sample?
A sample where every sampling unit has an equal chance of being selected
Explain how you would carry out a simple random sample
1) Assign each item a number
2) Use a random number generator (such as random function on a calculator or spreadsheet) to select numbers
3) The sampling units corresponding to these numbers become the sample
What are the advantages of a simple random sample?
+ Bias free
+ Easy and cheap
+ Unit have an equal chance of selection
What are the disadvantages of a simple random sample?
- Not suitable for a large population
- Requires a sampling frame (which may not be possible)
What is systematic sampling?
Required elements are chosen at regular intervals in ordered list.
Explain how you could carry out a systematic sample?
1) Assign each item a number
2) k = population size/sample size
3) Randomly select the first number using a random number generator between 1 and k
4) Then choose every kth item after this
What are the advantages of a systematic sample?
+ Simple, quick
+ Suitable for large populations
What are the disadvantages of a systematic sample?
- Need sampling frame
- Can introduce bias is the sampling frame is not random
What is stratified sampling?
Where the population is divided into groups (strata) and a simple random sample carried out in each group.
Explain how you would carry out a stratified sample
1) Calculate the proportion of each strata to be sampled (sample size/population) x strata size
2) Carry out a simple random sample for each strata:
i) Assign a number to each unit
ii) Use a random number generator (such as random function on a calculator or spreadsheet) to select numbers
iii) The sampling units corresponding to these numbers become the sample
What are the advantages of a stratified sample?
+ Reflects the structure of the population
What are the disadvantages of a stratified sample?
- Population must be classified into strata (not always possible)
- Has the same issues of a simple random sample
What is quota sampling?
A quota is set for each group in a population proportionally. Sampling units are then selected until the quota for each group is filled.
How would you carry out quota sampling?
1) Calculate the proportion of each strata to be sampled (sample size/population) x strata size
2) Select sampling units until each quota is filled.
What are the advantages of quota sampling?
+ No sampling frame is required
+ Allows a small sample to be representative of a population
+ Quick, easy and inexpensive
+ Easy comparison between different groups
What are the disadvantages of quota sampling?
- Non random sampling can introduce bias
- Population must be separated into groups which could be costly/inaccurate
- Non-responses are not recorded
What is opportunity sampling?
Sample taken from people who are available at time of study, who meet criteria.
What are the advantages of opportunity sampling?
+ Easy to carry out.
+ Inexpensive.
What are the disadvantages of opportunity sampling?
- Unlikely to provide a representative sample.
- Highly dependent on individual researcher.
