Sampling Design Notes Pre-College Math

How do we gather data? Surveys Opinion Polls Interviews Experiments Studies Observational Retrospective (past) Prospective (Future)

Definitions Population: The entire group of individuals that we want information about.

Census: A complete count of the population. Definitions Census: A complete count of the population.

Definitions Sample: A part of the population that we actually examine in order to gather information. (Use a sample to generalize the population.)

Definitions Sampling Design: Refers to the method used to choose the sample from the population.

Sampling Frame: A list of every individual in the population. Definitions Sampling Frame: A list of every individual in the population.

Definitions Random digit table: (just what it says…see below on your notes worksheet for an example).

Definitions Bias: Error Favors certain outcomes Anything that causes the data to be wrong May be attributed to the researchers, the respondent, or the sampling method.

Why would we not use a census all the time? 1) Not accurate 2) Very expensive 3) Perhaps impossible 4) If using destructive sampling, you would destroy the population.

Types of Sampling Designs Do Card Match 10 minutes. (Yes it is timed!) Be sure to check your answers with Mrs. Hodge, and then write them down in the blanks on your worksheet.

Identify the sampling design 1) The Educational Testing Service (ETS) needed a sample of colleges. ETS first divided all colleges into groups of similar types (Small public, small private, etc.) Then they randomly selected 3 colleges from each group. Stratified Random Sample

Identify the sampling design 2) A county commissioner wants to survey people in her district to determine their opinions on a particular law up for adoption. She decides to randomly select blocks in her district and then survey all who live on those blocks. Cluster Sampling

Identify the sampling design 3) A local restaurant manager wants to survey customers about the service they receive. Each night the manager randomly chooses a number between 1 & 10. He then gives a survey to that customer, and to every 10th customer after them, to fill it out before they leave. Systematic Random Sample

Sources of bias in Surveys These can cause bias in your sample. Cannot do anything with bad data

Sources of bias in Surveys 1) Voluntary Response: People chose to respond (Self selection) Usually only people with very strong opinions respond

Sources of bias in Surveys 2) Convenience Sampling: Ask people who are easy to ask Produces biased results (friendly or convenient)

Sources of bias in Surveys 3) Undercoverage: Some groups of the population are left out of the sampling process Ex: Use phonebook – leave out people unlisted numbers or without phones or cell phones.

Sources of bias in Surveys 4) Nonresponse: Occurs when an individual chosen for the sample can’t be contacted or refuses to cooperate. Telephone surveys 70% nonresponse rate.

Sources of bias in Surveys 5) Response Bias: Occurs when the behavior of respondent and/or interviewer causes bias in the sample. Wrong answers Ex: The interviewer may cause respondent to not give honest answers.

Sources of bias in Surveys 6) Wording of the Question: Wording can influence the answer that is given Connotation of words Use of “big” words or technical words may cause bias.

What is the source of bias? 1) Before the presidential election of 1936, FDR against Republican ALF Landon, the magazine Literary Digest predicting Landon winning the election in a 3–to–2 victory in a survey of 10 million people. George Gallup surveyed only 50,000 people and predicted that Roosevelt would win. The Digest’s survey came from magazine subscribers, car owners, telephone directories, etc. Undercoverage

What is the source of bias? 2) Suppose that you want to estimate the total amount spent by students on textbooks each semester at SMU. You collect register receipts from students as they leave the bookstore during lunch one day. Convenience Sampling or Undercoverage

What is the source of bias? 3) To find the average value of a home in Plano, one averages the price of homes that are listed for sale with a realtor. Undercoverage