If you are quoting several works published by the same author in the same year, they should be differentiated by adding a lower case letter directly, with no space, after the year for each item: Earlier evidence from Hills (1996a) found that…but later research conducted again by Hills (1996b) confirms… When several publications in the same year are referred to on a single occasion, or an author has made the same point in several publications, they can be referred to by using lower case letters: Hills (1996a; 1996b) has mentioned on numerous occasions that …