AP®StatisticsCollege BoardRevision NotesUnit 3: Collecting DataSampling Methods & BiasNon-random (Biased) Sampling Methods

Non-random (Biased) Sampling Methods (College Board AP® Statistics)

Revision Note

Mark Curtis

Expertise

Maths

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?

Advantages

Disadvantages

  • Useful with small samples that need to be representative of the population structure

  • Quick

  • Cheap

  • Does not require a list of all elements in the population

  • It is a non-random sampling method which makes it biased

  • Nonresponse bias (people may refuse to answer or be chosen)

  • Interviewer may have their own biases (e.g. only selecting individuals they like the look of!)

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

Advantages

Disadvantages

  • Useful with small samples

  • Quick

  • Cheap

  • Does not require a list of all elements in the population

  • It is a non-random sampling method which makes it biased

  • Nonresponse bias (people may refuse to answer or be chosen)

  • Interviewer may have their own biases (e.g. only selecting individuals they like the look of!)

  • Not likely to be representative of the population

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?

Advantages

Disadvantages

  • Quick

  • Cheap (volunteers often give information for free)

  • Does not require a list of all elements in the population

  • Good for platforms that have a high volume of users (to compensate for the low response rate)

  • It is a non-random sampling method which makes it biased

  • Voluntary response bias (only represents views of volunteers)

    • These may be strong opinions!

  • Low response rates (not many people respond)

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

    Next topic

    Did this page help you?

    Mark Curtis

    Author: Mark Curtis

    Mark graduated twice from the University of Oxford: once in 2009 with a First in Mathematics, then again in 2013 with a PhD (DPhil) in Mathematics. He has had nine successful years as a secondary school teacher, specialising in A-Level Further Maths and running extension classes for Oxbridge Maths applicants. Alongside his teaching, he has written five internal textbooks, introduced new spiralling school curriculums and trained other Maths teachers through outreach programmes.