The General Social Survey (GSS) is an example of a sample survey. It gathers in- formation by interviewing a sample of subjects from the U.S. adult population to provide a snapshot of that population. The study in Example 3 on student drug use in schools also used a sample survey.
2Most surveys of research about cell phones suggest that there is no convincing evidence yet of adverse radiation effects (e.g., D. R. Cox, J. Roy. Statist. Soc., Ser. A, vol. 166, pp. 241–246, 2003). The primary danger appears to be people using cell phones while driving!
Sample Survey
A sample survey selects a sample of subjects from a population and collects data from them. In the field of statistics a survey does not just indicate an opinion poll or a question- naire; it includes any information gathered from or about the subject.
A sample survey is a type of nonexperimental study. The subjects provide data on the variable or variables measured. There is no assignment of subjects to differ- ent treatments.
Most countries conduct a regular census. A census attempts to count the number of people in the population and to measure certain characteristics about them. It is different from a sample survey, which selects only a small part of the entire population.
In Article 1, Section 2, the U.S. Constitution states that a complete counting of the U.S. population is to be done every 10 years. The first census was taken in 1790. It counted 3.9 million people; today the U.S. population is estimated to be more than 300 million (308,745,538 by the 2010 U.S. census). Other than counting the population size, here are three key reasons for conducting the U.S. census:
j The Constitution mandates that seats in the House of Representatives be ap- portioned to states based on their portion of the population measured by the census. When the 1910 census was completed, Congress fixed the number of seats at 435. With each new census, states may gain or lose seats, depending on how their population size compares with other states.
j Census data are used in the drawing of boundaries for electoral districts.
j Census data are used to determine the distribution of federal dollars to states and local communities.
Although it’s the intention of a census to sample everyone in a population, in practice this is not possible. Some people do not have known addresses for the census bureau to send a census form. Some people are homeless or transient.
Because data are needed regularly on economic variables, such as the unem- ployment rate, the U.S. Bureau of the Census continually takes samples of the population rather than relying solely on the complete census. An example is the monthly Current Population Survey, which surveys about 50,000 U.S. house- holds and is the primary source of labor force statistics for the population of the United States, including the unemployment rate. It is usually more practical, in terms of time and money, to take a sample rather than to try to measure every- one in a population.
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For a sample survey to be informative, it is important for the sample to reflect the population well. As we’ll discuss next, random selection—letting chance de- termine which subjects are in the sample—is the key to getting a good sample.
U.S. Commerce Secretary Gary Locke stands beside a screen showing the country’s resident population during the 2010 Census presentation at the National Press Club in Washington.
4.1 Cell phones Consider the cell phone Study 3 described in Example 1.
a. Identify the response variable and the explanatory variable.
b. Was this an observational study or an experiment?
Explain why.
4.2 High blood pressure and binge drinking Many studies have demonstrated that high blood pressure increases the risk of developing heart disease or having a stroke. It is also safe to say that the health risks associated with binge drinking far outweigh any benefits. A study published in Heath Magazine in 2010 suggested that a combination of the two could be a lethal mix. As part of the study that fol- lowed 6100 South Korean men aged 55 and over for two decades, men with high blood pressure who binge drank even occasionally had double the risk of dying from a stroke or heart attack when compared to teetotalers with normal blood pressure.
a. Is this an observational or experimental study?
b. Identify the explanatory and response variable(s).
c. Does the study prove that a combination of high blood pressure and binge drinking causes an increased risk of death by heart attack or stroke? Why or why not?
4.3 Low-fat versus low-carb diet? One hundred forty-eight men and women without heart disease or diabetes enrolled in a study. Half of the subjects were randomly assigned to a low-carb diet 1640 g>d2, and the others were given a low-fat diet (630% of daily energy intake from total fat).
Subjects on the low-carb diet lost more weight after one year compared with those on the low-fat diet (an average
4.1 Practicing the Basics
of 8 pounds more). (L. A. Bazzano et al., Ann Intern Med 2014; 161(5): 309–318. doi: 10.7326/M14-0180)
a. Identify the response variable and the explanatory variable.
b. Was this study an observational study or an experimen- tal study? Explain.
c. Based on this study, is it appropriate to recommend that everyone who wishes to lose weight should pre- fer a low-carb diet over a low-fat diet? Explain your answer.
4.4 Experiments versus observational studies When either type of study is feasible, an experiment is usually pre- ferred over an observational study. Explain why, using an example to illustrate. Also explain why it is not always possible for researchers to carry out a study in an experi- mental framework. Give an example of such a situation.
4.5 Tobacco prevention campaigns A study published by www.tobaccofreekids.org concluded that prevention cam- paigns organized by tobacco companies were ineffective at best and even worked to encourage kids to smoke.
Nevertheless, tobacco companies spent $9.6 billion on such prevention campaigns in 2012 in the U.S.
a. Are the variables y = amount spent on tobacco com- pany prevention campaigns per year in the U.S. and x = annual number of young smokers positively or negatively correlated?
b. What, in your opinion, could be the real reason to- bacco companies have youth prevention campaigns?
4.6 Hormone therapy and heart disease Since 1976 the Nurses’ Health Study has followed more than 100,000
Section 4.1 Experimental and Observational Studies 187
Data were analyzed for 40,075 women. Rates of death were reduced in the screening group as compared to the historical screening group and in the nonscreening group as compared to the historical nonscreening group.
a. Is this an observational or experimental study?
b. Identify the explanatory and response variable(s).
c. Does the study prove that being offered mammography screening causes a reduction in death rates associated with breast cancer? Why or why not?
4.9 Experiment or observe? Explain whether an experiment or an observational study would be more appropriate to investigate the following:
a. Whether caffeine has an effect on long-term memory.
b. Whether multitasking in class affects the grades of business students.
c. Whether studying abroad tends to be associated posi- tively with international labor market mobility later in life for university graduates.
4.10 Baseball under a full moon During a baseball game between the Boston Brouhahas and the Minnesota Meddlers, the broadcaster mentions that the away team has won “13 consecutive meetings between the two teams played on nights with a full moon.”
a. Is the broadcaster’s comment based on observational or experimental data?
b. The current game is being played in Boston. Should the Boston Brouhahas be concerned about the recent full moon trend?
4.11 Seat belt anecdote Andy once heard about a car crash victim who died because he was pinned in the wreckage by a seat belt he could not undo. As a result, Andy refuses to wear a seat belt when he rides in a car. How would you explain to Andy the fallacy behind relying on this anec- dotal evidence?
4.12 Job opportunity in New York City Tina’s mother is ex- tremely proud that her daughter has a great job offer from a prestigious company located in Manhattan. However, she expresses concern when she learns that, based on sta- tistics collected from 2003 through 2015, 494 homicides on average are committed per year in New York City. Should her mother’s findings about the homicide rate drive Tina to reject the offer?
4.13 What’s more to blame for obesity? In a study published in the July 7, 2014, edition of the American Journal of Medicine, it was suggested that lack of exercise contrib- uted more to weight gain than eating too much. The study examined the current exercise habits and caloric intake of a sample of both males and females. (Source: http://www.
cbsnews.com/news/whats-more-to-blame-for-obesity-lack- of-exercise-or-eating-too-much/)
a. Was this an observational study or an experimental study? Explain why.
b. Identify the response variable and the explanatory variable(s).
c. Does this study prove that lack of exercise causes weight gain more often than eating too much?
d. It was reported that women younger than 40 are quite vulnerable to the risks of a sedentary lifestyle. Name nurses. Every two years, the nurses fill out a questionnaire
about their habits and their health. Results from this study indicated that postmenopausal women have a reduced risk of heart disease if they take a hormone replacement drug.
a. Suppose the hormone-replacement drug actually has no effect. Identify a potential lurking variable that could explain the results of the observational study.
(Hint: Suppose that the women who took the drug tended to be more conscientious about their personal health than those who did not take it.)
b. Recently a randomized experiment called the Women’s Health Initiative was conducted by the National Institutes of Health to see whether hormone therapy is truly helpful. The study, planned to last for eight years, was stopped after five years when analyses showed that women who took hormones had 30% more heart attacks. This study suggested that rather than reducing the risk of heart attacks, hormone replacement drugs actually increase the risk.3 How is it that two studies could reach such different conclusions? (For attempts to reconcile the studies, see a story by Gina Kolata in The New York Times, April 21, 2003.)
c. Explain why randomized experiments, when feasible, are preferable to observational studies.
4.7 Children of mothers with remitted depression A 2016 study (http://europepmc.org/abstract/med/26451509) in- vestigated parallels between affect recognition in mothers with remitted depression and their children. They examined two groups—a group of remitted depressed mothers and a group of healthy mothers. Mothers with remitted depression showed a higher accuracy and response bias for sadness.
The authors found corresponding results in their children.
Children of remitted depressed mothers appeared to be ex- posed to a sadness processing bias outside acute depressive episodes. This could make children of depressed mothers more vulnerable to depressive disorders themselves.
a. Identify the response variable and the explanatory variable.
b. Is this study an observational study or an experiment?
Explain.
c. Can we conclude that a child’s depressive disorder could be the result of having a mother with remitted depression? Explain.
4.8 Breast-cancer screening A study published in 2010 in the New England Journal of Medicine discusses a breast-cancer screening program that began in Norway in 1996 and was expanded geographically through 2005. Women in the study were offered mammography screening every two years. The goal of the study was to compare incidence-based rates of death from breast cancer across four groups:
1. Women who from 1996 through 2005 were living in countries with screening.
2. Women who from 1996 through 2005 were living in countries without screening.
3. A historical-comparison group who lived in screening countries from 1986 through 1995.
4. A historical-comparison group who lived in nonscreen- ing countries from 1986 through 1995.
3See article by H. N. Hodis et al., New England Journal of Medicine, August 7, 2003.
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b. What are reasons for taking the census at all?
c. The most commonly discussed characteristic learned from a census is the size of the population. However, other characteristics of the population are measured during each census. Using the Internet, report two such charac- teristics recorded during the 2010 U.S. census. (Hint: Visit the following website: www.census.gov/2010.census.) a lurking variable that might explain this risk of a
sedentary lifestyle for these younger women that in turn leads to little exercise and/or eating more.
4.14 Census every 10 years? A nationwide census is conducted in the United States every 10 years.
a. Give at least two reasons the United States takes a census only every 10 years.
4.2 Good and Poor Ways to Sample
The sample survey is a common type of nonexperimental study. The first step of a sample survey is to define the population targeted by the study. For instance, the Gallup organization (www.gallup.com) conducts a monthly survey of about 1000 adult Americans. Gallup then reports the percentage of those sampled who respond “approve” when asked, “Do you approve or disapprove of the way [the current president of the United States] is handling his job as president?” If the sampling is conducted properly, this provides a good estimate of the approval rate within the entire population. The population consists of all adults living in the United States.