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Wize University Statistics Textbook > Collecting Data & Sampling

Good Sampling

Statistical SamplingPrevious Section
Experimental and Observational StudiesNext Section
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Sampling Bias

A good sample is unbiased, meaning that the sample has the same attributes as the population of interest.

In other words, if a sample does not accurately represent the population of interest, then the sample is biased.

Different types of sampling biases

Non-random sampling bias
  • The sample must be randomly extracted from the population - You cannot cherry-pick!
  • You should not perform convenience sampling!
Voluntary response bias
  • Members decide if they if want to be part of the sample or not.
  • This biased sample will not be representative of the whole population.
  • Example People who are bored or have very strong opinions about a particular subject are more willing to participate (i.e. self-selection). You will end up with a biased sample full of bored and very opinionated people.
Nonresponse bias
  • If your sampling method is prone to low response (e.g. mailing surveys) as individuals that are chosen may be unable or unwilling to participate.
Undercoverage bias
  • This happens when a specific type of individuals in a population is not adequately represented in the sample.
  • Example Females make up only 29% of engineering majors at UCLA. If you only survey students near the men’s washroom at the Engineering Student Centre (i.e. convenience sampling), then females may likely be unrepresented in your sample.

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Recipe for a Good Sample

When determining the best sampling method, we need to consider the sample size, randomization of the sample, and potential sampling biases.

Size

  • A sample needs to be large enough according to the Central Limit Theorem but it does not need to be humungous.
  • There is no magic fraction of the population that we need to sample, but generally we do not sample over 10% of the population.
  • Additionally, we have no control over the population size.

Randomization

  • Obtaining a random sample is key.
  • Randomly selecting subjects in a sample minimizes the chance of obtaining a biased and non-representative sample.

Bias

  • Your sample needs to be free from bias.
  • A biased sample is not representative of the population of interest.

Practice: Sampling Bias

Which of the following could reduce sampling bias?

Practice: Good Sampling

A study on wellbeing and stress of university students is conducted. The researcher drew a random sample of 5 departments on campus. Then he randomly sampled 100 students within each department that was selected and handed them the link to his online survey. Ultimately, only 30% of the sample responded. Amongst those who responded, 120 of them say they feel stressed.

Which of the following is definitely true?
Extra Practice
Statistical Sampling
A marketing agency is interested in learning about the opinion of the general public about a new product that has been developed. In their study, they randomly call 200 home phone numbers and ignore people that do not have a home phone. Which of the following is least likely to occur?
Statistical sampling
Which statment about bias is/are TRUE?
i) Selection bias is significantly reduced when the sample size is larger
ii) Non-response bias results from random variation in the samples and will always be present
Statistical sampling
A textbook publisher received 5 shipments of 100 seemingly identical statistics books each. He chooses 2 shipments at random and checks 15 of the books randomly within each shipment, and found that 3 of these books have errors. Which of the following statement is TRUE?
Statistical Sampling
To determine the average household income in a very small town, a survey was mailed to 200 randonly selected homeowners in this town. Which of the following is most likely to occur?
Statistical sampling
One of the questions in a researcher's sample survey is: "Given one of the main ways germs are transmitted is through person-to-person contact, how often do you use hand sanitizer?"This will likely lead to?
Statistical Sampling
Which of the following statement is true?
Statistical Sampling
When you are increasing the sample size, which of the following is true?
Statistical sampling
A study on wellbeing and stress of university students is conducted. The researcher drew a random sample of 5 departments on campus. Then he randomly sampled 100 students within each department that was selected and handed them the link to his online survey. Ultimately, only 30% of the sample responded. Amongst those who responded, 120 of them say they feel stressed.
(c) Which of the following is definitely true?
Statistical Sampling
Which of the following could reduce sampling bias?
Population sampling
There are 600 Sephora locations in the United States. Christine randomly selected 40 stores, and then she randomly selected 100 customers from each of those stores. Less than 1% of her sample identified as heterosexual males. Based on her sample, the average spending at Sephora is $45. In addition, 340 customers in her sample revealed that they have VIB Rouge status, which the highest tier status that is reached when a customer spends $1,000 in a calendar year.
Which of the following is/are true? (Check all that applies.)