# Stats (Definitions)

Population
A whole set of items that are of interest
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Census
Observes / measures every member of the population
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Sample
A selection of observations taken from a subset of the population, which is used to find out information about the population as a whole.
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+: Should give a completely accurate result; -: Time consuming and expensive, hard to process a large quantity of data, cannot be used when testing destroys the item.
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+: Less time consuming and expensive, fewer people have to respond, less data to process; -: Data may not be as accurate, sample may not be large enough to give info about sub-groups of population
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Sampling units
Individual units of a population, which are often individually named/numbered to form a sampling frame.
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Types of sampling
Simple random, systematic, stratified
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Systematic sampling
Required elements are chosen at regular intervals from an ordered list.
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Simple random sampling
Samples are randomly chosen. In a sample of 'n', there is a '1/n' chance of anyone being picked.
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Stratified sampling
Population is divided into mutually exclusive strata e.g men/women , and a random sample is taken from each.
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+: Free of bias, easy and cheap to implement for small popns, each sampling unit has = chance of selection; -: Not suitable when population size / sample size is large, sampling frame needed. ..........................................................
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+: Simple and quick to use, suitable for large samples/ populations; -: Sampling frame needed, can introduce bias if sampling frame isn't random.
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+: Sample accurately reflects population structure, guarantees proportional representation of groups within populations; -: Popn. must be clearly classified into strata, selection from each stratum has same disadvantages as simple random sampling
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Types of non-random sampling
Quota, oppurtunity
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Quota Sampling
A researcher selects a sample that reflects the characteristics of the whole popn.
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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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+: Allows a small sample to be representative, no sampling frame required, quick easy & inexpensive, allows for easy comp. between diff groups; -: Can introduce bias, popn. must be divided into groups which can be £££/inaccurate, adding groups £££
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+: Easy to carry out, inexpensive; -: Unlikely to be representative, highly dependent on researcher
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## Other cards in this set

### Card 2

#### Front

Observes / measures every member of the population

Census

### Card 3

#### Front

A selection of observations taken from a subset of the population, which is used to find out information about the population as a whole.

### Card 4

#### Front

+: Should give a completely accurate result; -: Time consuming and expensive, hard to process a large quantity of data, cannot be used when testing destroys the item.

### Card 5

#### Front

+: Less time consuming and expensive, fewer people have to respond, less data to process; -: Data may not be as accurate, sample may not be large enough to give info about sub-groups of population