Observational studies
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As Yogi Berra, the former catcher and manager of the New York Yankees who is renowned for his humorous quotes, said, “You can observe a lot by watching.” Sometimes all you can do is watch. To learn how chimpanzees in the wild behave, watch. To study how a teacher and young children interact in a schoolroom, watch. It helps if the watcher knows what to look for. The chimpanzee expert may be interested in how males and females interact, in whether some chimps in the troop are dominant, in whether the chimps hunt and eat meat. Indeed, chimps were thought to be vegetarians until Jane Goodall watched them carefully in Gombe National Park, Tanzania. Now it is clear that meat is a natural part of the chimpanzee diet.
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Note that we have used the fact that a constant signal is a complex exponential with . If we express as , then the algebraic steps shows that y[n] can finally be expressed as a modified cosine signal plus a constant.
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At first, the observer may not know what to record. Eventually, patterns seem to emerge, and we can decide what variables we want to measure. How often do chimpanzees hunt? Alone or in groups? How large are hunting groups? Males alone, or both males and females? How much of the diet is meat? Observation that is organized and measures clearly defined variables is more convincing than just watching. Here is an example of highly organized (and expensive) observation.
Example 3 Do power lines cause leukemia in children?
Electric currents generate magnetic fields. So, living with electricity exposes people to magnetic fields. Living near power lines increases exposure to these fields. Really strong fields can disturb living cells in laboratory studies. What about the weaker fields we experience if we live near power lines? Some data suggested that more children in these locations might develop leukemia, a cancer of the blood cells.
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We can’t do experiments that deliberately expose children to magnetic fields for weeks and months at a time. It’s hard to compare cancer rates among children who happen to live in more and less exposed locations because leukemia is quite rare and locations vary a lot in many ways other than magnetic fields. It is easier to start with children who have leukemia and compare them with children who don’t. We can look at lots of possible causes—diet, pesticides, drinking water, magnetic fields, and others—to see where children with leukemia differ from those without. Some of these broad studies suggested a closer look at magnetic fields.
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One really careful look at magnetic fields took five years and cost $5 million. The researchers compared 638 children who had leukemia and 620 who did not. They went into the homes and actually measured the magnetic fields in the children’s bedrooms, in other rooms, and at the front door. They recorded facts about nearby power lines for the family home and also for the mother’s residence when she was pregnant. Result: no evidence of more than a chance connection between magnetic fields and childhood leukemia. Similar conclusions were reached by researchers at the University of Oxford in England who reviewed data from 1962–2008.
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“No evidence” that magnetic fields are connected with childhood leukemia doesn’t prove that there is no risk. It says only that a very careful study could not find any risk that stands out from the play of chance that distributes leukemia cases across the landscape. In other words, the study could not rule out chance as a plausible explanation for what was observed. Critics continue to argue that the study failed to measure some important variables or that the children studied don’t fairly represent all children. Nonetheless, a carefully designed observational study is a great advance over haphazard and sometimes emotional counting of cancer cases.
Response variable and observational study
A response is a variable that measures an outcome or result of a study. An observational study observes individuals and measures variables of interest but does not intervene in order to influence the responses. The purpose of an observational study is to describe some group or situation.
Statistics in Your World
You just don’t understand
A sample survey of journalists and scientists found quite a communications gap. Journalists think that scientists are arrogant, while scientists think that journalists are ignorant. We won’t take sides, but here is one interesting result from the survey: 82% of the scientists agree that the “media do not understand statistics well enough to explain new findings” in medicine and other fields.