Producing Data: Samples and Experiments Chapter 5

Warm up §A sociologist wants to know the opinions of employed adult women about government funding for day care. She obtains a list of the 520 members of a local business and professional women’s club and mails a questionnaire to 100 of these women selected at random. Only 48 questionnaires are returned. What is the population in this study? What is the sample?

Role of Sampling Design §An important goal of statistics is to answer questions using data with some guarantee that the answers are good ones. §An conclusion will be unreliable if the method of collecting data is flawed. §A poor design systematically favors certain outcomes or results and thus provides biased results.

Voluntary Response Design §Suppose the principal is interested in finding out if McCallum students think more trees should be planted. He makes an announcement and instructs students to come by his office to let him know if tree planting is an issue they support. §Discuss the following: l Will your results provide reliable information? l Define “voluntary response design” on white board.

Voluntary Response continued §A voluntary response sample consists of people who choose themselves by responding to a general appeal. §Voluntary response samples over represent people with strong opinions.

Convenience Sample Design §The principal is surprised to find most of the students coming in his office are in favor of the tree planting. Feeling that maybe his design may not have worked, he ventures into the hallways and starts asking students randomly. §Discuss the following: l Will your results provide reliable information? l Define “convenience sample design” on white board.

Random-random sample practice 1.simple random sample 2.convenience sample 3.cluster sample 4.voluntary response 5.systematic sample 6.stratefied sample 1.McCallum seniors 2.UT alumni 3.Time magazine subscribers 4.Texans 5.national pet stores 6.Austin middle school students

Cautions about sample surveys §T§The following are terms that describe potential problems while taking a sample. §u§undercoverage §n§nonresponse §r§response bias §w§wording of questions §D§Discuss and define each term in your group.

§Remember: sample results sometimes simply do not necessarily match the population. §undercoverage l the issue occurs when a sampling design misses a part of the population §nonresponse l the issue occurs when a significant part of the population refuses to participate in the survey

Cautions about sample surveys §response bias l the issue occurs when the person asking the question makes the respondent uncomfortable and possibly influence their answer §wording of questions l the issue occurs when a question is leading and attempts to persuade a respondent toward a particular answer

Identify potential problems §To obtain a sample of households, a television rating service dials numbers taken at random from telephone-directories. §Teen magazine sent a mail-in questionnaire to 500 randomly selected subscribers. One of the questions was the following: “Knowing that the cover price would likely increase, would you prefer the number of advertisements in the magazine to be limited.?”

Identify potential problems  For a survey of student opinions about high school athletic programs, a member of the school board obtains a random sample of students by listing all high school students and using a random number table to select 30 of them. After making phone calls last weekend, she notes six of the students said that they didn’t have time to participate in the survey.

Role of mathematics in sampling §Results will differ from sample to sample. This phenomenon is called sampling variability. §Since we deliberately use ranomization, the results obey the laws of probability allowing fairly consistent results. §The degree of accuracy can be improved by increasing the size of the sample.