It is important to build text–source integrity. That is, it should be easy for anyone reading the text to identify each source used to support each fact, and verify that it does so. This is the sole purpose of citations. Clarity does not require that you cite every sentence; several sentences may be supported by one tailing citation.
If the relationship between the cited facts and the source is complicated, then a detailed explanation should be included within the <ref>...</ref> statement ({{refn}} may also be used for nesting refs). Readability is often improved by citation bundling, where multiple citations and explanatory text are included within a single ref template. For example, take this discussion of an imaginary playwright's development:
Haplesses' first two plays,
Bicycle Ballet and
Airship Acrobats,
[1] were modest critical and commercial successes.
[2][3] In contrast, Hapless's third play,
Dragondrama, suffered a series of disasters.
[11] The fourth play,
The Calamity Theater, a light comedy, was widely seen as a fictionalized account of Hapless's experiences with
Dragondrama. It marked a notable departure from Hapless's early style, which had been criticized as over-earnest.
The Calamity Theater was praised for its subtle and thought-provoking humour,
[15] and Hapless was to use its gently ironic tone in all later works.
[16]
References
Gaffer, Clerk (1976). H. Hapless: the complete bibliography of works (Dullard's Library ed.). Moebius Press.
Wys, N. "New playwrights to watch". Cosmopolitan Cultural Review. Pretentious Press.
Nobby, S. "Airship Acrobats to close for good this winter". Cosmopolitan Cultural Review. Pretentious Press.
Legere, Brandon (1871). "Major fire in Conflagration Street". Lexicopolis Letters. No. 17.
Legere, Brandon (1874). "Fire guts construction site in Conflagration Street". Lexicopolis Letters. No. 17.
Game, Blame (1875). "Grand Reopening of Conflagration Street Theater goes much as expected". Lexicopolis Letters. No. 20.
"Bankruptcy filing for Incandescent Theater Company, Incorporated.", Lexicopolis bankruptcy filings, no. 456, 1877
Game, Blame (1875). "Chickenpox outbreak in Lexicopolis started in the theater scene". Lexicopolis Letters. No. 20.
Game, Blame (1875). "Weasel Solicitors collapses, international corruption investigation begins". Lexicopolis Letters. No. 21.
Corn, S. "The year's worst flop you never heard of". Cosmopolitan Cultural Review. Pretentious Press.
The theater burned down,[4] twice,[5] and was destroyed for a third time in a gas main explosion on the date of its Grand Reopening,[6] the theater company became involved in extensive litigation, eventually filing for bankruptcy,[7] every member of the children's chorus came down with chickenpox just before the opening,[8] several members of the production's firm of solicitors absconded to the East Indies with the meager take, leaving the cast and suppliers unpaid,[9] and the production was forced to close within a week.[10] Tamar, C. "Misfortune's favourite". Tamar's theater notes Review. Pretentious Press.
Game, Blame (1875). ""The Calamity Theater": better than the original". Lexicopolis Letters. No. 20.
Raize, Grugimp. "A theatrical disaster that is actually worth watching". Cosmopolitan Cultural Review. Pretentious Press.
formerly overearnest tone now improved, general praise:[12] just praise:[13][14] All three citations mention fictionalization. "Hapless, H. (playwright)". Encyclopedia Obscura (3rd ed.). Dusty Press. 1994.
The uncontroversial generalization "a series of disasters" is quite adequately supported by the eleventh citation, which bundles a description of seven individually-cited disasters (presumably the editor was unable to find a source which explicitly stated that the production had been a series of disasters, or simply wanted to provide additional detail for interested readers without unbalancing the passage). The end of the first sentence contains two different facts (a generalization that applies to each of two different plays), with a separate cite for each play. Which cite applies to which play need not be specified, as it can readily guessed by anyone reading the source titles. The fifteenth citation does briefly specify which sources support which facts, for easy reference, as it bundles three citations. This avoids repeating citations at the ends of successive sentences.
It is important to note that a large number of reference tags does not imply that a fact is more reliable. It is often a good rule to have two or three solid sources for controversial statements, or when the statement is supported by several different types of source, but these may be cited in one reference tag or several. Editors should not manipulate the number of square-bracketed tags in order to make a point. There is no universal "correct" number of citations. In some cases, you may want a long list of citations, bundled; in others, even one citation may be redundant. The correct number is whatever makes for the best article. Avoid citation overkill, adding citations which do not add to the article, and citation underkill, pruning citations to the detriment of the article.
Obscure sourcing
Prefer obvious verification to obscure verification. A reference can explain why the sourcing supports a statement, where needed.
For instance, it is obvious that the statement "The president spoke to representatives in Nairobi" is supported by a source saying "The president travelled to Nairobi in order to talk to representatives there"; everyone knows that "talk"="spoke" in this context. If the synonyms in question are "Stefan–Boltzmann distribution", "Planck curve" and "blackbody radiation spectrum", it may be less obvious. Terms may need to be defined in the reference, where someone trying to verify the information might not otherwise understand (example).
