Step Three – Choose the right search strings and limits
Once the right databases have been selected, the search string needs to be constructed. Although in most databases the order of the terms won’t make a difference, this must be thoroughly checked because the optimal order may differ between databases. In addition, limiting the search to just titles, abstract and title, or using a full text search will yield considerable differences.
Testing of search strings, the terms used and their order, and whether a truncated term and asterisk gives better results, is an iterative process. The construction of search strings will differ considerably between search engines, so it may not be possible to exactly duplicate the results using the same search string if the search engine is not known. Experienced personnel who understand the search engine requirements and the way each bibliographic database is indexed should undertake the construction of search strings, together with researchers who understand the subject area and can advise on alternative terms to ensure information is not missed because different terminology has been used.
Deciding the fields in which to search is crucial. Searching on only the”title” field may not pick up relevant content; nor will searching on the “abstract” field, particularly if the search is for some secondary endpoint or minor element of study results. Although searching on the full text will pick up a great deal of literature that is not useful and must be discarded, we prefer this approach because in our experience we have picked up many pivotal papers that would have otherwise been missed.
The table below shows the differences between adding terms involved in treatment (rows) or in searching on titles, abstracts or full text (columns). In this particular instance we wanted to determine how treatment responses differed in adult patients with psoriasis. This table is a sample for demonstration purposes only – so for brevity’s sake (and for confidential reasons) we have not included the full brief, methodology or study design.
|Search #||Search string||No of results searching titles only||No of results searching titles and abstracts||No of results searching full manuscripts|
|(psoriasis OR “psoriatic arthritis”) AND (outcome* OR respon* OR nonrespon* OR switch* OR heterogen* OR predict* OR mediat* OR moderat*) Limits: After 1 December 2001; Humans; Adults; Language: English||629||3348||5030|
|all((psoriasis OR “psoriatic arthritis”) AND (outcome* OR respon* OR nonrespon* OR switch* OR heterogen* OR predict* OR mediat* OR moderat* OR treat* OR medic* OR manag* OR therap*))Limits: After 1 December 2001; Humans; Adults; Language: English||2219||5441||9525|
Using the example above, when the first few pages of titles from the results of rows 1 and 2 were compared, to determine if any useful literature had been missed by not including the “treatment” related terms, it was evident that approximately half of the relevant literature had been omitted.
In addition, an even larger amount of literature is retrieved if the abstract or the full text of the article is searched for the relevant terms. When the first few pages of the results from columns 1 and 2 were examined, 15 relevant papers out of the first 100 had been missed by searching on titles only. As can be seen above, it is possible that this particular search performed on titles only (which incidentally is what the NICE guidelines for literature reviews recommend!) picked up less than a tenth of the potential literature. This may not be an issue if one is looking for a simple answer to the question “IS there a difference in treatment response in adults with psoriasis?”, but if one wants to understand HOW a difference manifests and in WHAT PROPORTION of patients, then the smaller number of citations may be insufficient for an accurate answer.
Please note, we are not saying here that as a rule of thumb, you will miss 50% of relevant literature by leaving out key words, or that you will miss 15% of the relevant literature by searching on titles only. This is our results for one search series in one therapy area on a particular day – our point is that you should clearly establish in your search strategy how much data you may be missing using your chosen method, and have made a judgement call on whether you can afford to miss it or not.