Search Filters: What's wrong or right with using them in systematic reviews?
Search filters (Hedges) are search strategies that have somehow been tested or studied to be a good way of finding records relevant to a concept (i.e. an illness, an intervention, a study design, an age group, etc.). The results of such a test or study could have been published or unpublished and could have been rigorous research or an institutional experiment.
We have many search filters, and the most famous ones are the Randomised Controlled Trials search filters recommended by Cochrane.
While many of us love to use such filters and feel like experts when we confidently suggest using them for our research or review, we must be cautious about what we wish for!
1. Nothing is 100%, except for our ignorance
Search filters are not necessarily bringing 100% of relevant records. They also get irrelevant records, and there is no search filter to confidently claim to find all the relevant records in the world of literature. While it is possible to develop a filter and tailor it to a gold standard selection of relevant records to find all of them, in the real world, such a filter may not function well as it will be biased in finding the already known records.
2. Taking a search block out of context
I will write another post about this, but in short, the search filter is only another search block for your search strategy. Taking it out of context could change its performance. Usually, the search filters have been developed based on a test and gold standard record collection from specific journals, published between certain years, for a particular database (i.e. MEDLINE). The performance will change if you change the context (journals, years, or database)!
Many search filters have a published paper in which they usually provide advice on how to use the filter properly and where are the limitations and considerations, without reading those papers it is impossible to get the most benefit from these filters. Even Harry Potter’s invisibility cloak came with an instruction: Use it well!