Sources of Bias 1. Voluntary response 2. Undercoverage 3. Nonresponse 4. Response bias 5. Convenience sample

Bias ERROR favors certain outcomes Anything that causes the data to be wrong! It might be attributed to the researchers, the respondent, or to the sampling method!

Sources of Bias things that can cause bias in your sample we cannot do anything with bad data

Remember – the way to determine voluntary response is: An example would be the surveys in magazines that ask readers to mail in the survey. Remember, the respondent selects themselves to participate in the survey! Remember – the way to determine voluntary response is: Self-selection!! People choose to respond Usually only people with very strong opinions respond

People with unlisted phone numbers Undercoverage some groups of population are left out of the sampling process People without phone numbers –usually low-income families Suppose you take a sample by randomly selecting names from the phone book – some groups will not have the opportunity of being selected! People with ONLY cell phones – usually young adults

Nonresponse Because of huge telemarketing efforts in the past few years, telephone surveys have a MAJOR problem with nonresponse! occurs when an individual chosen for the sample can’t be contacted or refuses to cooperate telephone surveys 70% nonresponse People are chosen by the researchers, BUT refuse to participate. NOT self-selected! This is often confused with voluntary response! One way to help with the problem of nonresponse is to make follow contact with the people who are not home when you first contact them.

Response bias Suppose we wanted to survey high school students on drug abuse and we used a uniformed police officer to interview each student in our sample – would we get honest answers? occurs when the behavior of respondent or interviewer causes bias in the sample wrong answers Response bias occurs when for some reason (interviewer’s or respondent’s fault) you get incorrect answers.

Wording Bias wording can influence the answers that are given The level of vocabulary should be appropriate for the population you are surveying Questions must be worded as neutral as possible to avoid influencing the response. wording can influence the answers that are given connotation of words use of “big” words or technical words – if surveying children then you should avoid complex vocabulary. – if surveying doctors, then use more complex, technical wording.

Convenience sampling Ask people who are easy to ask The data obtained by a convenience sample will be biased – however this method is often used for surveys & results reported in newspapers and magazines! Ask people who are easy to ask Produces bias results An example would be stopping friendly-looking people in the mall to survey. Another example is the surveys left on tables at restaurants - a convenient method!

Source of Bias? 1) Before the presidential election of 1936, FDR against Republican ALF Landon, the magazine Literary Digest predicted Landon winning the election in a 3-to-2 victory (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 – since the Digest’s survey comes from car owners, etc., the people selected were mostly from high-income families and thus mostly Republican! (other answers are possible)

Convenience sampling – easy way to collect data 2) Suppose that you want to estimate the total amount of money spent by students on textbooks each semester at Texas A&M. You collect register receipts for students as they leave the bookstore during lunch one day. Convenience sampling – easy way to collect data or Undercoverage – students who buy books from on-line bookstores are excluded.

(other answers are possible) 3) To find the average value of a home in Sugar Land, one averages the price of homes that are listed for sale with a realtor. Undercoverage – leaves out homes that are not for sale or homes that are listed with different realtors. (other answers are possible)

4) City police set up a road block in an area of the city to check cars for up-to-date registration and insurance. They check every car passing through that road. Identify the following: The population: Parameter of interest: Sampling frame: Sample: Sampling method: Any sources of bias: Cars in that city Registration and insurance List of all cars driven in that city Cars traveling in that road Cluster