What does the lowercase letter after the year mean when citing a work in a thesis? When should I add a lowercase letter to a cited author? What are the criteria for using it? Example: Johnson (2000a)
That indicates that there is more than one paper written by Johnson in 2000 that is being cited. If you go to the reference citations at the end of the text, you'll see that Johnson has at least two papers dating from 2000. The way to differentiate them is to add the letter. The second one will be 2000b, the third 2000c, and so on.