Some studies, such as Study 3 in Example 1, perform an experiment. Subjects are assigned to experimental conditions, such as when both cell phones are deacti- vated (“off” condition) or the right cell phone is activated (“on” condition), that we want to compare on the response outcome. These experimental conditions are called treatments. They correspond to different values of the explanatory variable.
Recall
From Section 1.2, the population consists of all the subjects of interest. We collect data on a subset of the population, which we call the sample.
From the beginning of Chapter 3, the response variable measures the outcome of interest. Studies investigate how that outcome depends on the explanatory variable. b
Experimental Study
A researcher conducts an experimental study, or more simply, an experiment, by assigning subjects to certain experimental conditions and then observing outcomes on the response variable (or variables). The experimental conditions, which correspond to assigned values of the explanatory variable, are called treatments.
For example, Study 3 used 47 participants as the subjects. In this experimental study, each participant was assigned to both treatments. These are the categories
Section 4.1 Experimental and Observational Studies 181
of the explanatory variable, whether the cell phone is off or on. The purpose of the experiment was to examine the association between this variable and the response variable—the amount of brain activity changes—to determine whether it increased.
An experiment assigns each subject to a treatment (or to both treatments as in the cross-over design employed in Study 3) and then observes the response.
By contrast, many studies merely observe the values on the response variable and the explanatory variable for the sampled subjects without doing anything to them. Such studies are called observational studies. They are nonexperimental.
Observational Study
In an observational study, the researcher observes values of the response variable and explanatory variables for the sampled subjects, without anything being done to the sub- jects (such as imposing a treatment).
In short, an observational study merely observes rather than experiments with the study subjects. An experimental study assigns each subject to one or more treatments and then observes the outcome on the response variable.
Identifying experimental versus observational
studies
b
Cell Phone Use
Picture the Scenario
Example 1 described three studies about whether relationships might exist between cell phone use and physiological activity in the human body. We’ve seen that Study 3 was an experiment. Studies 1 and 2 both examined the amount of cell phone use for cancer patients and for noncancer patients us- ing a questionnaire.
Questions to Explore
a. Were Studies 1 and 2 experimental or observational studies?
b. How were Studies 1 and 2 fundamentally different from Study 3 in terms of how the treatments were determined for each subject?
Think It Through
a. In Studies 1 and 2, information on the amount of cell phone use was gathered by giving a questionnaire to the sampled subjects. The sub- jects (people) decided how much they would use a cell phone and thus determined their amount of radiation exposure. The studies merely observed this exposure. No experiment was performed. So, Studies 1 and 2 were observational studies.
b. In Study 3, each subject was given both treatments (the “off” condi- tion and the “on” condition). The researchers did not merely observe the subjects for the amount of brain activity but exposed the subjects to both treatments and determined which treatment each would re- ceive first: Some received the call during the first PET scan and some during the second scan. The researchers imposed the treatments on the subjects. So, Study 3 was an experimental study.*
Example 2
*The researchers noted that “Results of this study provide evidence that acute cell phone expo- sure affects brain metabolic activity. However, these results provide no information about their relevance to potential carcinogenic effects (or lack thereof) from chronic cell phone use.”
182 Chapter 4 Gathering Data
Let’s consider another study. From its description, we’ll try to determine whether it is an experiment or an observational study.
Insight
One reason that results of different medical studies sometimes disagree is that they are not the same type of study. Experimental studies allow direct causal inference to be made, whereas observational studies do not allow causal inference. As we’ll see, there are different types of both observational and experimental studies, and some are more trustworthy than others.
c Try Exercises 4.1 and 4.3
1Study by R. Yamaguchi et al., reported in Journal of School Health, vol. 73, pp. 159–164, 2003.
Experiment or
observational study b
Drug Testing and Student Drug Use
Picture the Scenario
“Student Drug Testing Not Effective in Reducing Drug Use” was the headline in a news release from the University of Michigan. It reported results from a study of 76,000 students in 497 high schools and 225 middle schools nation- wide.1 Each student in the study filled out a questionnaire. One question asked whether the student used drugs. The study found that drug use was similar in schools that tested for drugs and in schools that did not test for drugs. For instance, the table in the margin shows the frequency of drug use for sampled twelfth graders from the two types of schools. In addition, the figure in the margin shows the conditional proportions of drug use for these students.
Questions to Explore
a. What were the response and explanatory variables?
b. Was this an observational study or an experiment?
Example 3
Drug Use
Drug Tests? Yes No Total
Yes 2,092 3,561 5,653
No 6,452 10,985 17,437
Think It Through
a. The study compared the percentage of students who used drugs in schools that tested for drugs and in schools that did not test for drugs.
Whether the student used drugs was the response variable. The ex- planatory variable was whether the student’s school tested for drugs.
Both variables were categorical, with categories “yes” and “no.”
For each grade, the data were summarized in a contingency table, as shown in the margin for twelfth graders.
b. For each student, the study merely observed whether his or her school tested for drugs and whether he or she used drugs. So this was an ob- servational study.
Insight
An experiment would have assigned schools to use or not use drug testing rather than leaving the decision to the schools. For instance, the study could have randomly selected half the schools to perform drug testing and half not to perform it and then, a year later, measured the student drug use in each school.
c Try Exercise 4.7
Section 4.1 Experimental and Observational Studies 183
As we will soon discuss, an experimental study gives the researcher more control over outside influences. This control can allow more accuracy in studying the association.