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APA Citation Style Guide (7th Edition)

When you have two or more documents with matching first author's surnames, you need to differentiate the two documents in the text.  Follow these basic steps.

Authors and Dates Matching

Step 1

Determine if the dates are the same. If two documents are written by the same author, for example, they may not have the same date.  If the dates are different, then simply providing the dates is enough to differentiate the two items. If the two items have the same date, head to the next step.

Example:

Jones (2012) first argued...

Jones (2018) further investigated the effects... 

Step 2

When two documents have the same date, but different secondary authors, provide as many authors' surnames as needed to differentiate the two items. This trick only works when you have multiple authors for a document.  If you have two items with the exact same dates and authors, head to the next step.

Example:

Lee, Harrison, Moore, Rogers, et al. (2008) studied...

Lee, Harrison, Moore, Walker, et al. (2008) provided evidence of... 

Step 3

When two documents have the exact same date and same authors, include a lowercase letter after the dates for both items. See the Step-by-Step Citation discussion for how this will look in the Reference list.

Example:

White (2017a) once described...

White (2017b) complemented his research with...

APA Manual pp.267-268, Sections 8.18 - 8.20