Bias On-Level Statistics

Problems with surveys (even when sampling methods are good) Undercoverage Some groups in the population are left out of the process of choosing a sample. Nonresponse Individual chosen for the sample can’t be contacted or does not cooperate *These problems may or may not cause bias.* Bias will result if the people left out are different, as a group, than the people included.

Error/Bias Sampling Error Response Bias Poor Question Wording Occurs because the sample rarely reflects the population perfectly. Can’t be avoided…we just have to account for it in our calculations (example: margin of error). Response Bias Occurs when a respondent does not give an accurate response. Causes: characteristics of the interviewer, lying, etc. Poor Question Wording One-sided, leading

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 cannot do anything with bad data

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

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!

People with unlisted phone numbers – usually high-income families 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 occurs when an individual chosen for the sample can’t be contacted or refuses to cooperate telephone surveys 70% nonresponse Because of huge telemarketing efforts in the past few years, telephone surveys have a MAJOR problem with 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 a follow-up 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 of the Questions The level of vocabulary should be appropriate for the population you are surveying wording can influence the answers that are given connotation of words use of “big” words or technical words – if surveying Podunk, TX, then you should avoid complex vocabulary. Questions must be worded as neutral as possible to avoid influencing the response. – if surveying doctors, then use more complex, technical wording.

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. 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 SMU. 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 included.

(other answers are possible) 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 – leaves out homes that are not for sale or homes that are listed with different realtors. (other answers are possible)