Writers of essays speculating about causes often rely on evidence from experts to support some causes and refute others. For college assignments, the instructor may require that certain kinds of sources be used and may even specify a minimum number of sources. But for most writing situations, you have to decide whether your sources are appropriate and sufficient. Using too few sources or sources that are too narrow in scope can undercut the effectiveness of your analysis. Consequently, it can be important to offer information from a number of sources and from sources that reflect a variety of areas of expertise.

Look, for example, at the sources Goode uses to support her causal analysis in “The Gorge-Yourself Environment” (pp. 426–30). Because her essay was originally published in a newspaper, she does not include a works cited list or parenthetical citations, but she does identify each of her sources by giving their credentials and academic affiliations: professors from a range of distinguished universities (such as Yale, the University of Pennsylvania, and the University of Minnesota) and different disciplines (such as psychology, nutritional science, marketing, and epidemiology). She uses as support for her analysis the specific findings of a number of academic research projects. In addition, she quotes the director of the Center for Science in the Public Interest advocacy group. The number and variety of expert sources adds to the credibility of Goode’s own analysis.

Similarly, student Sheila McClain uses a number of sources to support her causal analysis in “The Fitness Culture” (pp. 409–14). Because she is writing for a class, McClain includes both in-text citations and a list of works cited. Like Goode, she uses signal phrases (the author’s name and an appropriate verb, plus the author’s background where context is needed). McClain uses health club statistics from industry Web sites, but she does not rely solely—or even primarily—on industry or business sources. She also cites independent sources her readers are likely to find credible, such as the Pew Research Center, a university press book, and several newspaper and academic articles. McClain also refers to an interview she did with her exercise physiology professor. The number of sources, their authority, and their variety lend credibility to McClain’s own speculations.

As you determine how many and what kinds of sources to cite in your essay, keep in mind that readers of essays speculating about causes are more likely to be persuaded if the sources you rely on are neither too few nor too narrowly focused. If, when you begin to draft, you find that your evidence seems skimpy, you may need to do further research.