18 CHAPTER 1 Introduction to Statistics Distinguishing Between an Observational Study and an Experiment Determine whether each study is an observational study or an experiment. 1. Researchers study the effect of vitamin D3 supplementation among patients who were newly diagnosed with a viral infection. To perform the study, researchers give 2700 U.S. adults either a daily vitamin D3 supplement or a placebo for four weeks. (Source: U.S. National Library of Medicine) 2. Researchers conduct a study to determine how confident Americans are in the U.S. economy. To perform the study, researchers call 1019 U.S. adults and ask them to rate current U.S. economic conditions and whether the U.S. economy is getting better or worse. (Source: Gallup) SOLUTION 1. Because the study applies a treatment (vitamin D3) to the subjects, the study is an experiment. 2. Because the study does not attempt to influence the responses of the subjects (there is no treatment), the study is an observational study. TRY IT YOURSELF 1 The Pennsylvania Game Commission conducted a study to determine the percentage of the Pennsylvania elk population in each age and sex class. The commission captured and released elk during each year of the study and found an overall average of 16% branched bulls, 7% spike bulls, 56% adult cows, and 21% calves. Is this study an observational study or an experiment? (Source: Pennsylvania Game Commission) Answer: Page A35 Data Collection There are several ways to collect data. Often, the focus of the study dictates the best way to collect data. Here is a brief summary of two methods of data collection. • A simulation is the use of a mathematical or physical model to reproduce the conditions of a situation or process. Collecting data often involves the use of computers. Simulations allow you to study situations that are impractical or even dangerous to create in real life, and often they save time and money. For instance, automobile manufacturers use simulations with dummies to study the effects of crashes on humans. Throughout this course, you will have the opportunity to use applets that simulate statistical processes on a computer. • A survey is an investigation of one or more characteristics of a population. Most often, surveys are carried out on people by asking them questions. The most common types of surveys are done by interview, Internet, phone, or mail. In designing a survey, it is important to word the questions so that they do not lead to biased results, which are not representative of a population. For instance, a survey is conducted on a sample of physicians to determine whether the primary reason for their career choice is financial stability. In designing the survey, it would be acceptable to make a list of reasons and ask each individual in the sample to select their first choice. EXAMPLE 1
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