When doing research, it is easy and natural to find yourself using a number of sources written by the same author or authors: Many experts in specialized fields are quite prolific writers. But confusion can set in once you try to cite those sources, especially when names and dates repeat:
Jones, I., Jones, H., & Brody, M. (1957). This is our name, and this is our quest: I. Searching for the Holy Grail. Journal of Archeology, 55, 435–444. Jones, I., Jones, H., & Brody, M. (1957). This is our name, and this is our quest: II. Losing the Holy Grail. Journal of Archeology, 55, 445– 450. Jones, I., Ravenwood, M., & Williams, M. (1957). Crystal skulls are a girl’s best friend: Evidence for (space) aliens in North and South America? Parapsychological Reports, 51, 40–42. Jones, I., Scott, W., Round, S., & Brody, M. (1957). Properties of the Sankara Stones. Archeology Today, 22, 226–229. Jones, I., Williams, M., & Ravenwood, M. (1957). Swinging with the monkeys: Uncommon approaches to artifact retrieval. Archeological Methods, 88, 123–132.
The references above are presented in the correct order. Wait a moment: Alphabetically, the first two references are out of order (“Searching” should come after “Losing”—or, if you are being extremely picky, the second I of II)! However, as noted on page 182 of the Publication Manual of the American Psychological Association, sixth edition, “If the references with the same authors published in the same year are identified as articles in a series (e.g., Part 1 and Part 2), order the references in the series order, not alphabetically by title.” So the order is correct, but how can these two 1957 references of Jones, Jones, and Brody be differentiated when cited in text? Continue reading on page 182 of the Publication Manual: “Place lowercase letters— a, b, c, and so forth—immediately after the year, within the parentheses.” So our references become
Jones, I., Jones, H., & Brody, M. (1957a). This is our name, and this is our quest: I. Searching for the Holy Grail. Journal of Archeology, 55, 435–444. Jones, I., Jones, H., & Brody, M. (1957b). This is our name, and this is our quest: II. Losing the Holy Grail. Journal of Archeology, 55, 445– 450.
Note that letters are only used in citations when the years are the same and when all of the authors in the references are the same and in the same order. The remaining references in the initial group above do not take letters after their years, nifty as that trick is, because they can be differentiated by their authors and/or author order.
After the first in-text citation, the three-author sources with nonlettered years cannot be shortened to Jones et al., 1957, because too many references could be meant by that citation. That means the three-author references with nonlettered years are cited in full every time, and the four-author reference needs to include the first two authors in its citation to differentiate it:
Although Jones and colleagues have not always been successful at returning artifacts to museums, they have managed to track down—through hard work, determination, foolhardy risk taking, and luck—some of the greatest treasures of the world (Jones et al., 1957a, 1957b; Jones, Ravenwood, & Williams, 1957; Jones, Scott, et al., 1957; Jones, Williams, & Ravenwood, 1957).
Following these rules will help your documentation skills exceed those of Dr. Jones.
Source: APA Style Blog