Sometimes an author of a book, article or website will mention another person’s work by using a quotation or paraphrased idea from that source. The work that is mentioned in the article you are reading is called the primary source. The article you are reading is called the secondary source.
For example, suppose you are reading an article by Brown (2014) that cites information from an article by Snow (1982) that you would like to include in your essay. For the reference list, you will only make a citation for the secondary source (Brown). You do not put in a citation for the primary source (Snow) in the reference list. For the in-text citation, you identify the primary source (Snow) and then write "as cited in" the secondary source (Brown). If you know the year of the publication of the primary source, include it in the in-text citation. Otherwise, you can omit it. See below for examples.
Examples of in-text citations:
According to a study by Snow (1982, as cited in Brown, 2014), 75% of students believe that teachers should not assign nightly homework.
Note: If you don't have the publication date of Snow's article, you just omit it like this:
According to a study by Snow (as cited in Brown, 2014), 75% of students believe that teachers should not assign nightly homework.
In fact, 75% of students believe that teachers should not assign nightly homework (Snow, 1982, as cited in Brown, 2014).
Snow (1982, as cited in Brown, 2014) concluded that "nightly homework is a great stressor for many students" (p.34).
Example of Reference list citation:
Brown, S. (2014). Trends in homework assignments. Journal of Secondary Studies, 12(3), 29-38. http://doi.org/fsfsbit