Defining the Sample
Often, psychologists want their conclusions to apply to a particular population—all members of a given group. In our discussion of aggression, for example, we’ve been con-sidering a claim that potentially applies to all children. In other cases, we might be con-cerned with a narrower population—all 3-year-olds living in urban environments, or all patients suffering from schizophrenia, or all white-collar workers. Whatever the population we’re aiming at, though, we almost never draw data from the entire population. Instead, investigators study only a sample—a subset of the population they are interested in.
Why should we care about this distinction between sample and population? Imagine that an investigator studied aggression in a sample of children from wealthy families, and drew conclusions about boys and girls in general. These conclusions might be mis-leading because it’s possible that wealthy children behave differently from other chil-dren. Perhaps wealthy children are spoiled by their parents, so they’re less well behaved; or perhaps wealthy children are better educated, so they’re better behaved. In any case, these children might be different from their less wealthy peers; studying them might tell us a lot about this particular subset of our population, but it might not tell us much about others.
For these reasons, choosing a sample is a critical step in data collection. And, like most steps in a research project, the choice must be guided by the research question. If the question concerns a broad population (“How do boys in general differ from girls in general with regard to their aggression?”), then investigators need to select a sample that reflects this broader group. If the question is more specific (“How do Canadian boys differ from Canadian girls with regard to their aggression?”), the sample must be adjusted appropriately.
We also need to keep in mind that in any population, each person is different in some ways from every other. Some people are outgoing, some are more restrained; some are energetic, some are relaxed. How can we deal with this diversity and still make sure that our sample represents the broader group? A crucial tool for this purpose is randomsampling—a procedure in which every member of the population has an equal chanceof being picked to participate in the study. Random sampling helps investigators ensure that their sample mirrors the diversity of the overall population, so they can be more confident that their sample really does inform them about the properties of the popu-lation at large.
Random sampling is an important tool, but it’s not the only approach to sampling. In some settings, for example, a research question isn’t focused on what is common or typical in a population. Instead, researchers want to ask what’s possible in that popula-tion, or they want to examine directly how diverse the population is. For these purposes, the researcher might use the strategy of maximum variation sampling—a strategy of deliberately seeking out the unusual or extreme cases.
In addition, psychologists sometimes find it useful—and even necessary—to study single individuals rather than a broader sample. In these analyses, known as casestudies, investigators report data on one person—one case—in great detail. Casestudies have played an enormous role in the history of psychology. Many of Sigmund Freud’s claims were based on case studies—his analyses of the patients he saw in his clinical practice. Likewise, Jean Piaget’s influential theorizing about child development was based initially on the study of just three children—his own (Figure 1.6). Piaget and his followers then went on to test his claims with larger groups of chil-dren.
Case studies play a particularly important role in the study of the brain—specifically, in efforts to understand how the brain functions by examining cases of brain damage. One example is the case of a man called H.M. (1926–2008), whose memory deficits— resulting from neurosurgery for seizures—were severe and intriguing. H.M. was possibly the most studied person in the history of psychology, and his pat-tern of deficits offered crucial insights into how memory functions. Similarly, the case of Phineas Gage (1823–1860) has influenced our understanding of the functions of the brain’s frontal lobes. Phineas suffered damage to the frontal lobe when a construction accident forced a piece of iron through his head. His case was one of the first to inform psychologists about how damage to specific portions of the brain can change someone’s personality. Examples like these remind us that case studies are indeed an important part of the psychologist’s tool kit.
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