Statistics – Chapter 1 Data Collection 1.4 Sources of Error in Sampling Objectives: Understand how error can be introduced during sampling

Non-Sampling Errors Non-sampling errors are errors that result from the survey process Non-response of individuals selected to be in the survey Inaccurate responses Poorly worded questions Bias in the selection of individuals to be given the survey Bias – preference given to selecting some individuals over others

Sampling Errors Sampling errors are errors that result from using sampling to estimate information regarding a population. This type of error occurs because a sample gives incomplete information regarding the population. Example: 1936 Presidential Election Roosevelt (D) vs. Landon (R) Literary Digest mailed questionnaires based on list of subscribers, telephone directories, and automobile owners Lurking variables?

Sources of Non-Sampling Errors Frame – It is difficult to obtain a complete list of individuals in a population. If this is the case, part of the population is said to be underrepresented. Ex. Frame = all registered voters, who is underrepresented? Non-response – Either the individual doesn’t wish to respond or the researcher cannot contact them Ways to improve non-response: Callbacks Rewards Incentives

Sources of Non-Sampling Errors Interviewer Error – Trained interviewers are essential to obtain accurate information. A poor interviewer could make the subject uncomfortable with telling the truth. Be careful of survey results where the sponsor has a vested interest! Ex. A car dealer survey reports that 90% of past customers would buy another car from them. Misrepresented Answers – beware answers that misrepresent facts or are dishonest. Ex. Survey of recent college graduates reporting salaries

Sources of Non-Sampling Errors Data Checks – Data entry inevitably results in input errors. Data checks should be performed at every stage of statistical analysis to ensure accuracy. Questionnaire Design – Decisions about whether the questions should be open or closed.

Questionnaire Design Open Question: free to choose own response Ex. What is the most important problem facing America’s youth today? You want to phrase the question so the responses are similar. Too many different responses will not yield any results. Some surveys use open questions at the start, then use closed questions from the responses in the first stage of the survey.

Questionnaire Design Closed Question: Choose from a list of predetermined responses (forced choice). Ex. What is the most important problem facing America’s youth today? A) Drugs B) Violence C) Single-parent families D) Promiscuity E) Peer Pressure Limit the number of responses, results are easier to analyze This method doesn’t always allow the individual to answer the way they want to though

Sources of Non-Sampling Errors Wording of Questions - Questions should be balanced as to not represent one side of an argument. Ex. Do you oppose new gun legislation? Better Question: How do you feel about the new gun legislation? Order of Questions, Responses, and Words: should be rearranged periodically.