“Snowballing” in Systematic Literature Review

While doing systematic literature review, esp. in cases of complex evidence generation, we cannot rely soley on predefined, protocol driven search strategies, no matter how many databases are searched. Strategies that might seem less efficient (such as browsing library shelves, asking colleagues, pursuing references that look interesting, and simply being alert to serendipitous discovery) may have a better yield per hour spent and are likely to identify important sources that would otherwise be missed.

“Snowballing” also referred as ‘Pearl Growing’, ‘Bibliographic Search’ or ‘Citation Tracking’ is an important search method for identifying important articles relevant to your topic of interest.

It has been reported that up to 51% of references in a systematic review are identified by snowballing (1).  Yet, although experienced researchers regularly use the snowballing technique when conducting a systematic review, it is rarely reported in the methods section (2).

Snowballing refers to using the reference list of a paper or the citations to the paper to identify additional papers. Start with few articles that currently exist in or around your topic of interest; referred as ‘start set’; Once the start set is decided, it is time to conduct backward and forward snowballing.

Backward Snowballing:

means using the reference list to identify new papers to include. Following is a general step-wise approach that can be used:

• Go through the reference list and exclude papers that do not fulfil the basic criteria such as, for example, language, publication year and type of publication (if only considering peer reviewed papers).
• Remove papers from the list that have already been examined based on being found earlier
• The remaining papers are real candidates for inclusion for abstract/full text review
• If the paper is candidate for inclusion after having examined all information available in the paper being examined, then it is time to find the potentially new paper to include – check the reference list of this paper now.
• Continue that process until you cannot find any more relevant articles.

Forward snowballing:

refers to identifying new papers based on those papers citing the paper being examined. Facility known as ‘citation tracking’ available in large online databases such as Google Scholar; helps in this process

• Each candidate citing the paper is examined. The first screening is done based on the information provided in Google Scholar. If this information is insufficient for a decision, the citing paper is studied in more detail (abstract/FTA review)
• The approach to go through the papers is similar as to papers identified using backward snowballing.

Conclusion:

Snowballing, as a first search strategy, may very well be a good alternative to the use of database searches; however, this should not necessarily be seen only as an alternative rather this could be a complementary approach. In particular, it should be noted that snowballing is particularly useful for extending a systematic literature study.

#snowballing #SLR #systematicreview #HEOR #Literaturereview #knowledgesharing

References

(1) Greenhalgh T, Peacock R. Effectiveness and efficiency of search methods in systematic reviews of complex evidence: audit of primary sources. BMJ. 2005 Nov 5;331(7524):1064-5. Epub 2005 Oct 17. Review.

(2) Vassar M, Atakpo P, Kash MJ. Manual search approaches used by systematic reviewers in dermatology. J Med Libr Assoc. 2016 Oct;104(4):302-304. PubMed PMID: 27822152; PubMed Central PMCID: PMC5079492.