DATA COLLECTION METHODS Sampling

Class Objective After this class, you will be able to Use Non-probability Sampling Method Identify difficulties and disasters in sampling

Multistage Sampling Example: Using a combination of the sampling methods, at various stages. Example: Stratify the population by region of the country. For each region, stratify by urban, suburban, and rural and take a random sample of communities within those strata. Divide the selected communities into city blocks as clusters, and sample some blocks. Everyone on the block or within the fixed area may then be sampled. Copyright ©2011 Brooks/Cole, Cengage Learning

The Current Population Survey Unemployment rates in U.S. estimated each month based on interviews with sampled households as part of the Current Population Survey by the Bureau of Labor Statistics. Multistage Sample: Stage 1: country divided into ~2000 PSUs (primary sampling units) based on population and size; within each state, strata of PSUs created based on unemployment rates; one PSU is randomly selected from each of the strata. Stage 2: USUs (ultimate sampling units) randomly selected from each PSU. All households in the selected USUs visited. Stage 3: If number of households in Stage 2 still too many, a systematic sample of the households is obtained. Copyright ©2011 Brooks/Cole, Cengage Learning

Not Using a Probability Sample Responses from a self-selected group, convenience sample or haphazard sample rarely representative of any larger group. A Meaningless Poll “Do you support the President’s economic plan?” Results from TV quickie poll and proper study: Those dissatisfied more likely to respond to TV poll and it did not give the “not sure” option. Copyright ©2011 Brooks/Cole, Cengage Learning

Case Study The Infamous Literary Digest Poll of 1936 Election of 1936: Democratic incumbent Franklin D. Roosevelt and Republican Alf Landon Literary Digest Poll: Sent questionnaires to 10 million people from magazine subscriber lists, phone directories, car owners, who were more likely wealthy and unhappy with Roosevelt. Only 2.3 million responses for 23% response rate. Those with strong feelings, the Landon supporters wanting a change, were more likely to respond. (Incorrectly) Predicted a 3-to-2 victory for Landon. Copyright ©2011 Brooks/Cole, Cengage Learning

Case Study The Infamous Literary Digest Poll of 1936 Election of 1936: Democratic incumbent Franklin D. Roosevelt and Republican Alf Landon Gallup Poll: George Gallup just founded the American Institute of Public Opinion in 1935. Surveyed a random sample of 50,000 people from list of registered voters. Also took a random sample of 3000 people from the Digest lists. (Correctly) Predicted Roosevelt the winner. Also predicted the (wrong) results of the Literary Digest poll within 1%. Copyright ©2011 Brooks/Cole, Cengage Learning

Class Task What could go wrong in sampling? You have 5 minutes

Difficulties and Disasters in Sampling Some problems occur even when a sampling plan has been well designed. Using wrong sampling frame Not reaching individuals selected Nonresponse or nonparticipation Self-selected sample Convenience/Haphazard sample Copyright ©2011 Brooks/Cole, Cengage Learning

Using the Wrong Sampling Frame The sampling frame is the list of units from which the sample is selected. This list may or may not be the same as the list of all units in the desired “target” population. Example: using telephone directory to survey general population excludes those who move often, those with unlisted home numbers, and those who cannot afford a telephone. Solution: use random-digit dialing. Copyright ©2011 Brooks/Cole, Cengage Learning

Not Reaching the Individuals Selected Even with a properly selected sample, researchers might not reach the desired units. Telephone surveys tend to reach more women. Some people are rarely home. Others screen calls or may refuse to answer. Quickie polls: almost impossible to get a random sample in one night. Copyright ©2011 Brooks/Cole, Cengage Learning

Nonresponse, Volunteer Response, and Nonparticipation Even with a properly selected sample, individuals cannot be contacted or refuse to participate. Volunteer responses not likely to represent the entire sample, and leads to nonresponse or nonparticipation bias. Response rates should be reported in summaries. Copyright ©2011 Brooks/Cole, Cengage Learning

Which Scientists Trashed the Public? “82% (of scientists) trashed the media, agreeing with the statement ‘The media do not understand statistics well enough to explain new findings.’ ” Science (Mervis, 1998) Science Poll 1400 professionals (in science and in journalism). Only 34% response rate among scientists. Typical respondent was white, male physical scientist over age of 50 doing basic research. Respondents represent a narrow subset of scientists  inappropriate to generalize to all scientists. Copyright ©2011 Brooks/Cole, Cengage Learning

Homework Reading: Assignment: Chapter 5 Exercise 5.57 – 5.60 Chapter 5 – p. 166 - 170