Consider a Source's Author(s)
Writers use sources for a variety of purposes: to support a point, to illustrate a range of positions on an issue and to show that they are not alone in their opinions, to name a few. With rare exceptions (such as when you are illustrating divergent opinions), the authors you cite should be reliable and trustworthy.
A Note on Field Sources: When conducting field research, you are actually selecting the sources, or "authors," of your information. In particular, in the case of an interview, you select a recognized "authority" to provide you with specific data and informed opinions on your subject.
Similarly, but in a more general sense, you select a group of people to which you intend to distribute a questionnaire or a particular setting or environment that you intend to observe.