Digital Object Identifiers (DOIs)
APA Style emphasizes DOIs when citing journal articles. Many modern scholarly (peer-reviewed) journals use DOIs. Consequently, when you see a DOI in a list of references in a book or article, it is most likely a scholarly source. It also appears to be more current because older scholarly articles do not have them.
Parenthetical References (in-text) Sec. 6.19:
(Last name of author, Year of publication, p. page number) - Example: (Fontane, 2015, p. 285).
If there are two authors with the same last name, start the citation with the first initial of the author's first name.
URLs and DOIs? Sec. 6.31 - 6.32:
APA Style asks for as much information as possible; but Uniform Resource Locators (URLs) are often changing in electronic format. A small group of publishers organize their content with the Digital Object Identifiers (DOI) system to keep a persistent identifier for scholarly articles. APA prefers DOIs to URLs. Sometimes DOIs are difficult to locate. Some database aggregators, such as Ebscohost and JSTOR, change URLs or do not have DOIs for all of their content. In such cases, you may cite the database (http://www.jstor.org) instead.
Double or Single Spacing? Sec. 8.03:
APA strongly indicates double spacing for everything including, text, captions, quotations, and footnotes.
You may use single spacing or 1.5 spacing for tables and figures.
Spacing after punctuation marks Sec. 4.01:
Use one space after commas, colons, semicolons, periods, parts of a reference citation, and initials in names.
Use two spaces after the end of each sentence.