Skip to Main Content
Hong Kong Baptist University Hong Kong Baptist University Library

Citing Sources (APA, MLA, ...): Citation FAQs

Need help with citation? Start here!

What is a DOI? Do I need to include it in my citation?

Q: What is a DOI? Do I need to include it in my citation?

A digital object identifier (DOI) is a unique character string that can be assigned any document or work. They are designed to be permanent identifiers that will take you directly to the document no matter where it exists, even if its current URL changes.

Any type of document can have a DOI. All major publishers assign DOIs to their journal articles, and smaller publishers increasingly do so as well. While not as common, books and book chapters can also be assigned a DOI.

In general, you should include the DOI in your citation when it is available. They are preferred over URLs that may change over time. However, they are not a requirement and indeed some sources may not have a DOI. Do not spend inordinate amounts of time looking for one if it is not immediately obvious.

Exactly how the DOI should be presented depends on the citation style you are using:

APA style and Chicago style:

Present the DOI as a persistent URL, e.g.

MLA style:

Present the DOI as follows - doi:10.1353/pmc.2000.0021

How do I cite a source that I found in another source, but haven't consulted directly myself?

Q: How do I cite a source that I found in another source, but haven't consulted directly myself? (i.e. an indirect citation)

As a matter of good scholarship, you should make every effort to consult sources directly. After viewing the source yourself, you may find that you disagree with the interpretation in the first source you found, or you may even find that it has been misquoted. Should you have trouble getting hold of the original, try using our Interlibrary Loan service.

If it is really impossible to get hold of the original, you can make an indirect citation. The exact manner depends on which style you are using.

APA Style

You should cite the source in text as follows:

Smith has argued that ... (as cited in Wong, 2013, p. 254).


It has been argued that ... (Smith, as cited in Wong, 2013, p.254).

In your reference list, include only the source that you have seen yourself (Wong's work in the example above). Do not include the original work in the reference list (Smith in this case).

MLA Style

You should cite the source in text using qtd. (for "quoted") as follows:

Smith has argued that ... (qtd. in Wong 254).

As with APA, do not include the original in your list of works cited.

Chicago Style

Section 14.260 of the Chicago Manual (17th Edition) deals with citations taken from secondary sources. As with APA and MLA style, the Chicago Manual discourages such citations "since authors are expected to have examined the works they cite".

If it is unavoidable, an indirect citation can be made. In contrast with APA and MLA, you should list both the original and secondary source in your footnote:

     1. Louis Zukofsky, "Sincerity and Objectification," Poetry 37 (February 1931): 269, quoted in Bonnie Costello, Marianne Moore: Imaginary Possessions (Cambridge, MA: Harvard University Press, 1981), 78.

Do I need to include a retrieval date for all of the sources that I find online?

Q: Do I need to include a retrieval date for all of the sources that I find online?

In the early days of Internet sources, it was a common requirement to provide a retrieval date for information accessed online. It was recognized that such information might change over time. Gradually the major citation styles have recognized that the retrieval date is of limited value, and cannot be verified by the reader.

Below is a summary of the advice given by the latest editions of the major citation styles:

APA Style

According to the Seventh Edition of the publication manual (section 9.16) a retrieval date is only needed for sources that are inherently designed to change (e.g. dictionary entry, Twitter profile, Facebook page). The date indicates to your reader that the version they access may be different to the one that you used.

If you need to include a retrieval date, place it before the URL:

Retrieved March 16, 2020, from https://xxxxx

MLA Style

According to the MLA Handbook (Eighth Edition) the date of access is an optional element and it is therefore up to you as to whether or not to include it. However, it may be crucial to include it "if the source provides no date specifying when it was produced or published." (p. 53)

See our MLA citations page for examples of how to include the access date.

Chicago Style

The Chicago Manual notes that the value of the self-reported access date is limited as it cannot be verified and in any case previous version will often be unavailable to the reader. Chicago therefore does not require the access date "unless no date of publication or revision can be determined from the source" (section 14.12 of the Seventeenth Edition of the Manual).