Chapter 2 1

Chapter 2

Samples, Good and Bad

Chapter 2 2

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 3

Thought Question 2

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 4

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 5

Thought Question 4

A survey on poverty and welfare included the following question: “Do you agree with the popular notion that government policy should attempt to assist those individuals who have had the misfortune to end up living in poverty by providing them with much needed financial assistance until they can get back on their feet?” Based on the wording, do you think the author of this question was looking for support or opposition to welfare programs? Explain.

Chapter 2 6

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 7

Bad Sampling Plans

Convenience sampling– selecting individuals who 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 8

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 9

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 10

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 on pg. 550 of text, or

Random Number Table on pg. 189 of the supplement)– computer software

Chapter 2 11

Simple Random Sampling

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.

Example: Courses with Statistics Prerequisite

Page 550 of textbook:Pick a line and column at random: suppose we get line 111, column 3Random numbers: 605 130 929 700 412 712

Page 189 of supplement:Pick a line and column at random: suppose we get line 6, column 2Random numbers: 092 507 965 673 211 040

…

…

Chapter 2 12

Key Concepts

Bias Convenience Sampling Voluntary Response Sampling Simple Random Sampling