A number of different style guides are available.
APA 5th EditionEdit
Where the author is not identified (as in the case of a wiki), begin the reference with the title of the document. Also mark the title in italics. Where the article has no volume number, edition number, or date of publication (as in the case of a wiki), nenote no date (n.d.). Then present the date you retrieved the article (especially because wiki articles change all the time). Finally, give them the complete web address.
Factor - SPSS Base. (n.d.). Retrieved December 8, 2005, from http://spss.wikicities.com/wiki/Factor_-_SPSS_Base