Chapter 1 Sampling - Year 1 Flashcards
What is a population?
A population consists of all the items that we are interested in
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?
Sampling units of a population are individually named or numbered to form a list called the sampling frame
What is a census?
When information about the entire population is measured
What is a sample?
A small selection of the population
What are the advantages of a census?
It is completely accurate
What are the disadvantages of a census?
Time consuming
Expensive
Can’t be used when testing involves destruction
There are large volumes of data to proses (impractical)
What are advantages of a sample?
Less expensive
Less time consuming
Less data to process (more practical)
What are the disadvantage of a sample?
Data may not be accurate
Sample may be too small to be represent small sub-groups of the population
What are the different ways of random sampling?
Simple random
Systematic sampling
Stratified sampling
Explain how you would do a simple random sample
Allocate a number between 1 and x to each pupil
Use a random number generator to select y about of numbers
The students corresponding to those numbers become the sample
The assigning people numbers may have already be done for you in the question you would need to say use the … eg odifjcjfmekwoeodkdkdkdk
Explain how you would do a systematic sample
Population ÷ sample wanted = z
Assign each person a random number between 1 and y
Randomly select a number between 1 and z as a starting point
Then select that person and every zth person after that
The assigning people numbers may have already be done for you in the question you would need to say use the … eg odifjcjfmekwoeodkdkdkdk
Explain how you would do a stratified sample?
First you work out the number of people you will sample of each group
(Sample ÷ total) X size of stratified sample
(Sample ÷ total) X size of stratified sample
…
Explain how you would sample inside each group. If the question doesn’t say use simple random sample as it is easier. Make sure to be using exact info. Eg from the class register. Number 1 - y. Random number generator
How could you randomly select people without a random number generator?
Rolling a dice
What are the advantages of using a simple random sample?
Bias free
Easy
Cheap
What are the disadvantages of simple random sample?
Not suitable when the population size is large as smaller minority groups may be missed
May not be an accurate representation of the population
A sampling frame is needed
What are the advantages of systematic sampling?
Simple
Quick
Suitable for large samples and populations
What are the disadvantages of systematic sampling?
Can introduce bias if the sample is too small and patters emerge
A sampling frame is needed
What is a strata?
When you divide the populations in different groups
What are the advantages of stratified sampling?
Sample accurately reflects population structure
Guarantees proportional representation between different groups
What are the disadvantage of stratified sampling?
Sample frame is needed
Population must be clearly separated into different strata
Selection from each stratum is subjected to disadvantages from simple and systematic sampling
What is Quota sampling?
When the population is divided into different groups according to different characteristics so that a population is represented
Interviews then select the sampling unit until the quota is filled
What are the advantages of quota sampling?
Allows a small sample to be representative of a population
No sampling frame reauired
Relatively quick
Easy
Inexpensive
What are the disadvantages of quota sampling?
Non random sampling can introduce bias
There needs to be clear groups
The different groups may be not easily defined or rude
What types of sampling are non random?
Quota sampling
Opportunity sampling
What is opportunity sampling?
A sample taken from the people who are available at the time who need the criteria
What are the advantages of opportunity sampling?
Easy to do
Cheap
What are the disadvantages of opportunity sampling?
Unlikely to provide a representative sample
It is dependent on the researcher
Why may a random sample not be able to be used?
A sampling frame can’t be created
How do you do a opportunity sample?
You stand outside a place and you interview the first people who come buy who meet your criteria
How do you do a quota sample?
You need to calculate how big each quota is. Use the formula ((n.population ÷ Total population) X n.sample)
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What is the difference between stratified and quota sampling?
In a stratified sample the people selected within each strata are randomly selected but in a quota sample the sample will be the first n people who the researcher meets
