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Writing your thesis and conducting a literature review

Search strings

The next step is to create your search strings. Each search string will represent a theme/concept of your research. Search strings are a combination of keywords that are synonymous or related terms, along with acronyms and variant spellings. Your aim is to capture as much of the relevant literature as possible and search strings will help you do that.

There are a number of tools and search operators that you can use to improve the efficacy of your search strings.

  • Acronyms - If your keyword is also known by an acronym then both the full spelling and acronym should be searched, e.g., (“Armoured Fighting Vehicle” OR AFV).
  • Wildcard - If your keyword can be spelt differently, e.g., UK/US variants, then use a wildcard to capture both variations, e.g., defen?e will return papers with both defense and defence spellings.
  • Truncation - If your keyword could have several variant endings, then truncate it, e.g., technolog* will return papers with technology, technological, technologies, etc.
  • Phrase-searching – if your keyword has two or more words, then you should enclose the keywords within double quotation marks. If you do not, the database/search engine will put an invisible ‘AND’ operator between each word, e.g., systems engineering will be read by the database/search engine as systems AND engineering. This means that both systems and engineering must appear in the search results, but not necessarily together in the context of systems engineering. To ensure systems engineering is returned in your results as a phrase, search for “systems engineering”.

Note: Not all databases search using the same syntax, check the search tips of the database to check they accept wildcards, truncation, phrase-searching, etc. and adjust your search string accordingly.

Combining your keywords and search strings: search operators

  • Boolean operators – are an effective way to combine your keywords and search strings. There are three main operators:
    • OR - is used to combine synonyms, related terms, acronyms, etc. within search strings, e.g., (“Armo?red Fighting Vehicle*” OR AFV OR tank* OR “Infantry Fighting Vehicle*”) will return results where any one of these keywords appears. Using the OR operator expands your search results.
    • AND – is used to combine search strings of different concepts/themes, e.g., (“Armo?red Fighting Vehicle*” OR AFV OR tank* OR “Infantry Fighting Vehicle*” OR IFV OR “Armo?red Personnel Carrier*” OR APC) AND (protect* OR securit* OR IED OR "improvised explosive device*") will return results where any one keyword from the first search string and any one keyword from the second search strings appears in the document. Using the AND operator will narrow down your search results.
    • NOT/AND NOT – is used to eliminate keywords that you do not wish to appear in your search results, e.g., tank* NOT water will exclude results which contain the keyword water. Use this operator with caution, for example it is unlikely to exclude all results that mention water.

Watch this Boolean operators video to learn more about using them to connect your keywords and other commands such as quotation marks, question marks and asterisks which will all help you to improve your search results.

  • Proximity operators – enable you to search for keywords or search strings within a set number of words of each other. This can be useful when you wish there to be context between your keywords/search strings but using the phrase option is too restrictive. Not all databases/indexes offer a proximity search, and the operators differ on each platform, so make sure to check the search tips before you begin. An example from Scopus:
    • Covid W/3 (PPE OR “personal protective equipment”) will return results where Covid appears within three words of PPE or “personal protective equipment”, regardless of the order of the words.
    • Covid PRE/3 (PPE OR “personal protective equipment”) will return results where Covid appears first within three words of PPE or “personal protective equipment”.

Further guidance from Kings College London on advanced search techniques.

Search strategy

A search strategy is simply your search strings combined to address each of your review questions, to answer your overarching research topic. Search strings are usually combined with a Boolean operator or a proximity operator, as described above.