Sampling Flashcards
Population definition
The whole set of items that are of interest.
Give two examples of populations
The items manufactured by a factory
All the people in a town
What can be obtained from a population?
Information
Information obtained from a population is known as …
Raw data
Census definition
A census observes or measures every member of a population.
Sample definition
A selection of observations taken from a subset of the population which is used to find out information about the population as a whole.
What are the advantages of a census?
• It should give a completely accurate result.
What are the 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
Large data sets are produced which:
- Are difficult to manage without computers
- And can include missing pieces of data
What are the advantages of a sample?
- Less time consuming and expensive than a census
- Fewer people have to respond
- Less data to process than in a census
What are the disadvantages of a sample?
- 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 will the size of the sample affect?
The validity of any conclusions drawn
What does the size of the sample depend upon?
The required accuracy and available resources
Generally, the larger the sample …
The more accurate it is, but you will need greater resources
In what cases will you need a larger sample?
If the population is very varied, you need a larger sample than if the population were uniform
Why can different samples lead to different conclusions?
Due to the natural variation in a population
What are individual units of a population called?
Sampling units
What are sampling units?
Individual units of a population
Often sampling units of a population are individually named or numbered to form a list called a …
Sampling frame
What is a sampling frame?
Often sampling units of a population are individually named or numbered to form a list called a sampling frame.
Example 1
A supermarket wants to test a delivery of avocados for ripeness by cutting them in half.
a) Suggest a reason why the supermarket should not test all the avocados in the delivery.
The supermarket tests a sample of 5 avocados and finds that 4 of them are ripe. They estimate that 80% of the avocados in the delivery are ripe.
b) Suggest one way that the supermarket could improve their estimate.
a) Testing all avocados is a census, which will destroy all the product so that there is nothing to sell.
b) To improve estimate:
- Take a larger sample
- Sample numerous deliveries
- Sample over time
What are the three methods of random sampling?
- Simple random sampling
- Systematic sampling
- Stratified sampling
What are the key features of random sampling?
- Every member of the population has an equal chance of being selected.
- The sample should therefore be representative of the population.
- Random sampling also helps to remove bias from a sample.
How can a simple random sample be carried out?
- You need a sampling frame (usually a list of people or things)
- Each person or thing is allocated a unique number and a selection of these numbers is chosen at random.
What are the two methods of choosing the numbers in a simple random sample?
- Generating random numbers (using a calculator, computer or random number table)
- Lottery sampling
One method of choosing the numbers in a simple random sample is lottery sampling. How can lottery sampling be carried out?
The names/unique numbers of the members of the sampling frame could be written on identical tickets/cards and placed into a hat.
The required number of tickets/cards would then be drawn out blindfolded.
What is systematic sampling?
In systematic sampling, the required elements are chosen at regular intervals from an ordered list.
The first person to be chosen should be chosen at random.
What is stratified sampling?
In stratified sampling, the population is divided into mutually exclusive strata and a random sample is taken from each.
What are the 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
What are the disadvantages of simple random sampling?
- Not suitable when the population size or the sample size is large
- A sampling frame is needed
What are the advantages of systematic sampling?
- Simple and quick to use
* Suitable for large samples and large populations
What are the disadvantages of systematic sampling?
- A sampling frame is needed
* It can introduce bias if the sampling frame is not random
What are the advantages of stratified sampling?
- Sample accurately reflects the population structure
* Guarantees proportional representation of groups within a population
What are the 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.
What are the two types of non-random sampling?
Quota sampling
Opportunity sampling
What is quota sampling?
In quota sampling, an interviewer or researcher selects a sample that reflects the characteristic of the whole population.
How can quota sampling be carried out?
- 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.
- As an interviewer, you would meet people, assess their group and then, after interview, allocate them into the appropriate quota.
- This continues until all quotas have been filled. If a person refuses to be interviewed or the quota into which they fit is full, then you simply ignore them and move on to the next person.
What is opportunity sampling?
Opportunity sampling consists of taking the sample from people who are available at the time the study is carried out and who fit the criteria you are looking for.
Opportunity sampling is sometimes called …
Convenience sampling
Give an example of opportunity sampling
This could be the first 20 people you meet outside a supermarket on a Monday morning who are carrying shopping bags.
What are the advantages of quota sampling?
- Allows a small sample to still be representative of the population
- No sampling frame required
- Quick, easy and inexpensive
- Allows for easy comparison between different groups with a population
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
- Increasing scope of study increases number of groups, which adds time and expense
- Non-responses are not recorded as such
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
