The abbreviation et al. means and others. It is most often used when a work you are citing in your paper has multiple authors. When a work has two authors, cite both names every time the reference occurs in the text.
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APA Style Contacts. A digital object identifier (DOI) is a unique alphanumeric string assigned by a registration agency (the International DOI Foundation) to identify content and provide a persistent link to its location on the Internet. The publisher assigns a DOI when your article is published and made available electronically.
http://www.apastyle.org/learn/faqs/what-is-doi.aspx
When you are doing an APA style reference for a source that does not list a copyright date, you use (n.d.) as the date of publication. It stands for No Date. For more information on APA citation, visit the library's APA Help page. Thank you for using ASK US. Contact your Library/ARC for other questions.
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1 When a work has two authors, cite both names every time the reference occurs in the text.  When a work has three, four, or five authors, cite all the authors the first time the reference occurs; in subsequent citations, include only the surname of the first author followed by et al.
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We repeat MEMBERS of the LEGISLATIVE ASSEMBLY. APA stands for American Psychological Association. We repeat American Psychological Association MLA, according to The OWL At Purdue, is Modern Language Association. APA is American Psychological Association. MLA stands for<b>M</b>embers of the <b>L</b>egislative <b>A</b>ssembly. We repeat <b>M</b>EMBERS of the <b>L</b>EGISLATIVE <b>A</b>SSEMBLY. APA stands for <b>A</b>merican <b>P</b>sychological <b>A</b>ssociation.
http://www.answers.com/Q/What_is_MLA_and_APA_format
Learning APA Style. A digital object identifier (DOI) is a unique alphanumeric string assigned by a registration agency (the International DOI Foundation) to identify content and provide a persistent link to its location on the Internet. The publisher assigns a DOI when your article is published and made available electronically.
http://www.apastyle.org/learn/faqs/what-is-doi.aspx
The abbreviation et al. means and others. It is most often used when a work you are citing in your paper has multiple authors. Here are the guidelines from the Publication of the American Psychological Association (6th ed.), page 175: When a work has two authors, cite both names every time the reference occurs in the text.
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Some of the more commonly used examples of abbreviations are listed below. Edition. For the edition of a book. Editor(s) When a book includes editors rather than authors. This means 'and others'. This is used in intext citations when there are 3 or more authors. NOTE: stop only after 'al.', not after 'et'.
http://guides.lib.monash.edu/citing-referencing/apa-abbreviations
What is a digital object identifier, or DOI? A digital object identifier (DOI) is a unique alphanumeric string assigned by a registration agency (the International DOI Foundation) to identify content and provide a persistent link to its location on the Internet.
http://www.apastyle.org/learn/faqs/what-is-doi.aspx
For example: Mathews' use of...(2010, n. p.). NOTE: Instructions for electronic sources without page numbers. If page numbers are not given use approximate page number (p. 3 of 9); or paragraph number for short text (para. 2); or the heading given in the source for the particular section. For example:
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what does n.d. stand for in apa
In APA  n.d. stands for No Date. In APA, N.D. stands for No Date.