Transcript Chapter 2
Chapter 2 Samples, Good and Bad Chapter 2 1 Thought Question 1 Popular magazines often contain surveys that ask their readers to answer questions about hot topics in the news. Do you think the responses the magazines receive are representative of public opinion? Explain why or why not. Chapter 2 2 Thought Question 2 The Cable News Network (CNN) often asks its viewers to call the network with their opinions on certain political issues, like whether or not they favor current foreign policy. Do you think the results of these polls represent the feelings of the general population? Do you think they represent the feelings of all those watching CNN at the time? Explain. Chapter 2 3 Thought Question 3 Researchers in health and nutrition often study the effectiveness of new supplements by providing subjects with both the new supplement and a “fake” supplement made to look like the real thing. Do you think the results would be biased if the person providing the products to the participants knew which was which? Chapter 2 4 Thought Question 5 Suppose you access an online listing of all courses at your institution, alphabetized by department, to determine what proportion of all courses have a statistics course as a prerequisite. If you decide to sample 50 courses in order to get a representative sample of courses, how would you select them? Would it be appropriate to simply select the first 50 courses listed? Chapter 2 5 Bad Sampling Plans Convenience sampling – selecting individuals that are easiest to reach Voluntary response sampling – allowing individuals to choose to be in the sample Both of these techniques are biased – systematically favor certain outcomes Chapter 2 6 Convenience Sampling Sampling mice from a large cage to study how a drug affects physical activity – lab assistant reaches into the cage to select the mice one at a time until 10 are chosen Which mice will likely be chosen? – could this sample yield biased results? Chapter 2 7 Voluntary Response To prepare for her book Women and Love, Shere Hite sent questionnaires to 100,000 women asking about love, sex, and relationships. – 4.5% responded – Hite used those responses to write her book Moore (Statistics: Concepts and Controversies, 1997) noted: – respondents “were fed up with men and eager to fight them…” – “the anger became the theme of the book…” – “but angry women are more likely” to respond Chapter 2 8 Simple Random Sampling Each individual in the population has the same chance of being chosen for the sample Each group of individuals (in the population) of the required size (n) has the same chance of being the sample actually selected Random selection: – “drawing names out of a hat” – random number table (see Table A in back of the text, or Random Number Table in back of the supplement) – computer software Chapter 2 9 Simple Random Sampling Example: Courses with Statistics Prerequisite Suppose there are 800 courses at an institution, alphabetized by department (and numbered 001-800), and you decide to randomly select 50 of them to determine what proportion of all the courses have a statistics course as a prerequisite. Use a random number table to select which 50 courses to sample. Table A in back of the textbook: Pick a line and column at random: suppose we get line 111, column 3 Random numbers: 605 130 929 700 412 712 … Random Number Table in back of the supplement: Pick a line and column at random: suppose we get line 6, column 2 Random numbers: 092 507 965 673 211 040 … Chapter 2 10 Key Concepts Bias Convenience Sampling Voluntary Response Sampling Simple Random Sampling Chapter 2 11