The TARCiS statement comprises ten evidence-based recommendations and four research priorities. It provides guidance for systematic reviewers on when and how to conduct citation searching and on how to report it.

TARCiS is intended to be used by researchers, systematic reviewers, information specialists, librarians, editors, peer reviewers, and others who are conducting or assessing citation searching methods.

(For rationale and explanation texts, click on the headings.)

Recommendation 1

The following terminology should be used to describe search methods that exploit citation relationships:

• "Citation Searching" as an umbrella term,
• "Backward Citation Searching" to describe the sub-method retrieving and screening cited references,
  • "Reference List Checking" to describe the sub-method retrieving and screening cited references by manually reviewing reference lists,
• "Forward Citation Searching" to describe the sub-method retrieving and screening citing references,
• "Co-cited Citation Searching" to describe the sub-method retrieving and screening co-cited references,
• "Co-citing Citation Searching" to describe the sub-method retrieving and screening co-citing references,
• "Iterative Citation Searching" to describe one or more repetition(s) of a search method that exploits citation relationships, and
• "Seed References" to describe relevant articles which are known beforehand and used as a starting point for any citation search.

Recommendation 2

For "difficult-to-search-for" systematic search topics, backward and forward citation searching should be seriously considered as supplementary search techniques.

Recommendation 3

For "easier-to-search-for" systematic search topics addressed by a highly sensitive search, backward and forward citation searching are not explicitly recommended as supplementary search techniques. Reference list checking of included records can be used to confirm the sensitivity of the search strategy.

Recommendation 4

Backward and forward citation searching as supplementary search techniques should be based on all included records of the primary search, i.e., all records that meet the inclusion criteria of the review after full-text screening of the primary search results. Occasionally, it can be justified to deviate from this recommendation and either use further pertinent records as additional seed references or only a defined sample of the included records.

Recommendation 5

Backward citation searching should ideally be conducted by screening the titles and abstracts of the seed references as provided by a citation index. Screening titles as provided when checking reference lists of the seed references can still be performed.

Recommendation 6

Using the combined coverage of two citation indexes for citation searching to achieve more extensive coverage should be considered if access is available. This is especially meaningful if seed references cannot be found in one index and reference lists were not checked.

Recommendation 7

Before screening, the results of supplementary backward and forward citation searching should be deduplicated.

Recommendation 8

If citation searching finds additional eligible records, another iteration of citation searching should be considered using these records as new seed references.

Recommendation 9

Stand-alone citation searching should not be used for literature searches that aim at completeness of recall.

Recommendation 10

Reporting of citation searching should clearly state

• the seed references (along with a justification should the seed references differ from the set of included records from the results of the primary database search),
• the directionality of searching (backward, forward, co-cited, co-citing),
• the date(s) of searching (which may differ between rounds of iterative citation searching) (not applicable for reference list checking),
• the number of citation searching iterations (and possibly the reason for stopping if the last iteration still retrieved additional eligible records),
• all citation indexes searched (e.g., LENS.ORG, Google Scholar, Scopus, citation indexes in Web of Science) and, if applicable, the tools that were used to access them (e.g., Publish or Perish, citationchaser),
• if applicable, information about the deduplication process (e.g., manual/automated, the software or tool used),
• the method of screening (i.e., state whether the records were screened in the same way as the primary search results or, if not, describe the alternative method used), and
• the number of citation searching results in the right column box of the PRISMA 2020 flow diagram for new or updated systematic reviews which included searches of databases, registers and other sources.

Research Priority 1

The effectiveness, applicability, and conduct of indirect citation searching methods as supplementary search methods in systematic reviewing require further research (including retrieval of additional unique references, their relevance for the review and prioritization of results).

Research Priority 2

Further research is needed to assess the value of citation searching. Potential research topics could be:

• influence of citation searching on results and conclusions of systematic evidence syntheses,
• topics or at least determinants of topics where citation searching is likely/not likely to have additional value, or
• economic evaluation of citation searching to assess the cost and time of conducting citation searching in relation to its benefit.

Research Priority 3

Further research is needed to assess the best way to perform citation searching. Potential research topics could be:

• optimal selection of seed references,
• optimal use of indexes and tools and their combination to conduct citation searching,
• methods and tools for deduplication of citation searching results,
• subjective influences on citation searching (e.g., experience of researcher, prevention of mistakes), or
• reproducibility of citation searching.

Research Priority 4

Further research is needed to reproduce existing studies: Any recommendations in this Delphi that are based on only 1-2 studies require reproduction of these studies in the form of larger, prospectively planned studies that grade the evidence for each recommendation and propose additional research where the grade of evidence is weak.

As of November 2023