Non-random (Biased) Sampling Methods (College Board AP® Statistics)
Study Guide
Written by: Mark Curtis
Reviewed by: Dan Finlay
Quota sampling
What is quota sampling?
Quota sampling is when a population is first divided into distinct groups (just like with stratified sampling)
Elements are then chosen for the sample from the different groups
The number of elements from each group in the sample (the quotas) often correspond to the proportion of the population in that group
(again, like stratified sampling)
However, elements are chosen from each group for the sample using a non-random method until each quota is filled
e.g. asking people who walk by on a street
This is why quota sampling is a non-random (biased) sampling method
What advantages and disadvantages of quota sampling do I need to know?
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Worked Example
A judo tournament has 30 beginner members, 50 intermediate members and 20 advanced members.
Leo wants to sample 10 members so decides to wait until the end of the tournament then ask the first 3 beginners, first 5 intermediates and first 2 advanced members that leave.
(a) Indicate the type of sampling being used.
Answer:
Quota sampling
(b) Describe a disadvantage of Leo's sampling method.
Answer:
Leo's method is a non-random method
This means there will be a bias towards the judo members who happen to be the first to leave the tournament (those who leave later will not be represented)
Convenience sampling
What is convenience sampling?
Convenience sampling is when a researcher samples whoever happens to be easily accessible at that particular time and/or place
It is also called opportunity sampling
The researcher decides who to include
They may selects individuals in a non-random (biased) way
e.g. whoever walks past them
What advantages and disadvantages of convenience sampling do I need to know?
Advantages and disadvantages are similar to quota sampling, but quota sampling has the added benefit of splitting the population into groups first
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Voluntary response sampling
What is voluntary response sampling?
A voluntary response sample is one in which individuals can chose (volunteer) whether or not to participate
It is also called a self-selected sample
Examples include
optional online surveys
call-in shows
What advantages and disadvantages of voluntary response sampling do I need to know?
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