Suppose paper 1 establishes that roses are red, and cites paper 2 for the statement that violets are blue. Apart from the blueness of violets, no other results from paper 2 are relevant for paper 1, nor for my own essay.
How should my essay refer to these two assertions?
I see three possible options:
- Citing the primary source for each statement: Roses are red. Violets are blue.
- Citing each relevant source for each statement: Roses are red. Violets are blue.[1,2]
- Downgrading the second statement to a secondary citation: Roses are red. 2 had earlier established that violets are blue.
Is there a generally accepted preference for one of these?
It really depends on the context and the reasons why you feel that sharing such assertions contributes to your own essay.
Generally, the first option you mentioned is best, which is citing each assertion according to its original source: Roses are red. Violets are blue.
In some cases, if you have no access to source 2 cited in 1, it may be acceptable to cite only 1, particularly if 1 is a meta-analysis or a review of existing assertions: 1 notes that roses are red and violets are blue.
Similarly, if you are using a direct quote that was itself directly quoted in source 1, it is generally acceptable to simply explicitly make note of this if it is difficult to track down the original source 2: "violets are blue"[2, cited in 1]
Your second option could falsely imply that both source 1 and 2 independently demonstrated that violets are blue (which they do not in your example), and your third option misleadingly cites 1 when you have already cited 2 in the sentence, which is confusing to the reader.