Some review teams use 'snowballing' to find relevant studies that were not retrieved by their search strategy. This is the process of citation searching, both for articles that were cited in included studies (backward snowballing) and for articles that have cited the included studies (forward snowballing).
Snowballed references are reported in the PRISMA 2020 flow diagram in the section 'Identification of studies via other methods'
References identified by snowballing can be imported into Covidence in the usual way. In order to track and report these studies accurately, make sure that the first round of screening is complete before starting work on the snowballed set.
Next, do the following:
- Import the snowballed references into the Title and abstract screening stage.
- Click "All" at the top. This selects all the newly imported references.
- Click 'Tags' and type Snowballing, then click 'Create new'.
- You should now see all new references have been tagged with 'Snowballing'.
When the title and abstract screening is complete for this set, check the Full text review section to see how many references made it through. Make a note of this number for your records.
When the full text screening is complete, run an export as follows:
- Click on 'Export'
- In the References box, select 'Excluded' for stage and 'CSV' for format.
- Once you open the downloaded file, you should be able to see if any 'Snowballing' tagged studies were excluded and for what reason.
Thank you to Covidence users Michelle R Demetres and Andy Hickner for sharing this useful tip in their blog post.