A citation signal indicates how a writer views the relationship of a citation to some statement being made. If no signal is used, then the reader may assume that the citation supports the statement. Some examples of citation signals are: A citation is a credit or reference to another document or source which documents both influence and authority. ...
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See, e.g.:
These signal that the cited authority directly supports a proposition. They further indicate that other authorities also could have been cited for the same proposition, but no purpose would be served by citing them all because their citations would be duplicative. E.g., from the latin exempli gratia, means "for the sake of example".
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This signal indicates that the cited authority provides useful background information about a given point.
This signals (for the Latinconfer, meaning "compare") that the cited authority states a proposition different from that stated by the person citing to the authority, but that the cited authority's proposition is sufficiently analogous to lend support.
Contra:
This signals that the cited authority contradicts a given point: opposition.
MOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOoooooooooooo cf. ... Jump to: navigation, search Latin is an Indo-European language originally spoken in the region around Rome called Latium. ... Opposition may refer to a number of topics: astronomical opposition political opposition parliamentary opposition Opposition to a patent, see for instance Opposition procedure before the European Patent Office This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ...
From what I've seen, anecdotal and field-dependent though it might be, entering the citation as you read the paper is more the exception than the rule.
While it's almost certainly true that citation copying occurs, I'm taking issue with their assumption of strict causality without which their estimates are biased.
Also some individuals tend to remember those few cases when they copied a citation to a paper, which they have read, while forgetting numerous cases when they copied citations to papers, which they haven't read.