Define population
a population is the whole set of items that are of interest
Define census
a census observes or measures every member of a population
Define sample
a is a selection of observations taken from a subsect 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
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 are individual units of a population known as?
sampling units
What is a sampling frame?
sampling units of a population are individually named or numbered to form a list called a sampling frame
How does the size of a sample affect the validity of any conclusions drawn?
generally, the larger the sample, the more accurate it is
What does the size of the sample depend on?
depends on the required accuracy and available resources
When may you need a larger sample?
if the population is varied, you need a larger population than is the population is uniform
–> different samples can lead to different conclusions due to the natural variation in a population
Define simple random sampling
a simple random sample of size n is one where every sample of size n has an equal chance of being selected
Define systematic sampling
in systematic sampling, the required elements are chosen at regular intervals from an ordered list
Define stratified sampling
In stratified sampling, the population is divided into mutually exclusive strata (males and females for example) 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 as it is potentially time consuming, disruptive and expensive
- 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 advantages of stratified sampling?
- sample accurately reflects the population structure
- guarantees proportional representation groups within a population
What are disadvantages of stratified sampling?
- population must be clearly classified into distinct data
- selection within each stratum suffers from the same disadvantages as simple random sampling
Define quota sampling.
an interviewer or researcher selects a sample that reflects the characteristics of the whole population
What are two types of non-random sampling?
- quota sampling
- opportunity sampling
Define 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
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Key facts and skills - Key HW5
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Key facts and skills - Key HW25
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Key facts and skill - Key HW 45
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Algebraic and geometric facts9
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Proof2
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Graph transformations6
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Trigonometric identities and equations11
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Modelling assumptions28
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Mechanics - SUVAT10
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1a. Statistics - Data collection33
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1b. Statistics - The large data set16
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2. Statistics - Measures of location and spread16
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3. Representations of data12
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4. Correlation0
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5. Probability0
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6. Statistical distributions0
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7. Hypothesis testing0
