What is firmly established cannot be uprooted. What is firmly grasped cannot slip away.Lao Tsu
Reference to a secondary source
These guidelines are based on the following sources:
https://apastyle.apa.org/style-grammar-guidelines/citations/secondary-sources by APA Style
https://guides.lib.monash.edu/citing-referencing/vancouver-intext by Monash University Library
A secondary source, is a source that is cited or quoted in another source. The most common example is a article that cites another article. In the example below Buttle (2009) and Langlois (1992) are both cited by Mejtoft (2010). In other words, none of this information is derived by Mejtoft (2010) but are results by the original authors and the sources cited. Hence, it is Buttle (2009) and Langlois (1992) that should be read and cited in your work.
In scientific writing, and generally in most writings, it is important to actually read the original source. This is (most often) the only way that you can make a reference to the source in your writings.
Tip of the day! DO NOT MAKE REFERENCE TO SECONDARY SOURCES DUE TO LAZINESS!!
However, in some cases it is not possible to get hold of the original source. It might be out of print or not accessible. The most common scenario is that you have read the content and understood the essence of the original source (that cannot be found) in a secondary source.
In this case, the most important thing is that the secondary source have a high credibility. It is important the the secondary source is of extremely high quality and should (most often) be e.g., a peer-reviewed scientific publication. If you believe that the credibility of the secondary source is high enough it is possible to add a reference to the secondary source quoting the original source. You should, however, never base core/important parts of your writings on secondary sources.
Below are some examples on how to write a text referring to a secondary source.
<Original Authors> argue that .... (as cited in <Secondary Authors>) e.g. Simpson (1997, as cited in Mejtoft, 2008) argues that facts are meaningless. There are indications that facts are meaningless (Simpson, 1997, as cited in Mejtoft, 2008) or Simpson (as cited in ) argues that facts are meaningless. There are indications that facts are meaningless (Simpson as cited in )
The reference that is put in the reference list is, in this case, the secondary source (i.e., only the source that you have actually read!). In the example above it is Mejtoft (2008) that is a post in the reference list. The original source (Simpson, 1997) should not be put into the reference list.
Reference to a review article
In many cases it is possible to find very nice review articles (i.e., articles that compile data from previous published research to e.g. give an overview). In other words, the actual data is (usually) not new, but there might be new conclusions in the review article based on the compiled data.
When referencing a review article it is important to decide whether the review article should be referenced or if a reference should be done to one (or several) of the articles that are included in the review. If you are making a reference to a specific conclusion then the reference should be given to the specific article (that in included in the review) where the author(s) have made this specific conclusion. However, the final data/conclusions in a review article might also (in most cases) be regard as a primary source. In other words, the author(s) of the review article have, e.g., compiled and made conclusions bases on many articles and (in some cases) their own results. If the conclusion of the compiled data is the source you are referencing, the reference should be given to the review article as an ordinary reference.
This is an honest way of writing the reference that also indicate that this is a review article:
In a review by Mejtoft (2007) ... In the 50 articles reviewed by Mejtoft (2007) ... or In a review by Mejtoft  ... In the 50 articles reviewed by Mejtoft  ...
If you are looking for information on how to write references to personal communication: Reference to personal communication
(First published by Thomas Mejtoft: 2019-03-19; Revised: 2021-04-20; Last updated: 2022-03-21)