This section will cover basic principles needed to construct search filters for use on Internet search engines or on online databases. It includes general considerations relating to the choice of search terms and to the use of operators to define the links between the terms.

Search efficiency

The choice of search terms (words) and the way they are combined is very important, but other factors also affect the efficiency of a search. The effectiveness of a search can be measured in terms of recall and precision. These ratios vary between databases, between searches, and between search engines.

Recall ratio = relevant records retrieved/total relevant records (i.e. how comprehensive the search is)
Precision ratio = relevant records retrieved/total records retrieved (i.e. how accurate the search is)

Recall and precision are inversely related, so broadening the search (e.g. by including synonyms) will increase the recall, but at the cost of decreasing precision, because the probability of retrieving irrelevant material increases. Narrowing the search (by using specific terms or spellings) will increase precision, but at the risk of not retrieving some potentially very relevant material.

They will be affected by the capabilities of the search language (operators) that can be used. In the case of databases, they will also be affected by the indexing system and for search engines, by the algorithms used to assign relevancy.

If the recall ratio is expressed as a percentage of all relevant documents that exist rather than the total relevant records in the database being searched, it will also be affected by the coverage of the database. This will depend on which journals and other sources are monitored. Likewise, recall will be affected by the number of web sites indexed by a search engine and the depth to which these sites are indexed.

As a result, there may be great differences in the results obtained from different databases or search engines. It is important to bear this in mind, in order to search effectively it is better to use more than one database/search engine.

Choice of search terms

Appropriate search terms for documents relevant to the Three Rs are listed in Planning a search, so are not specifically covered here. Searching for information on the Three Rs, requires combining Three Rs search terms with search terms related to the scientific topic of interest.

Important factors in creating a search include:
• case specificity
• spelling
• synonyms and related words.
• operators used to link the search terms (see below)

Case specificity – Some search systems are case-specific for upper case letters. In these instances, the use of capital letters where appropriate, for example for acronyms and proper names, may reduce the number of irrelevant documents retrieved.

Spelling – Correct spelling of search terms is vital, because search software identifies exact matches. To maximise the chance of finding relevant documents it is sometimes necessary to use both British and American spellings, such as: anaesthesia, anesthesia, haemoglobin, hemoglobin.

Synonyms – It is also useful to include synonyms and related words which could be used by authors, for example the phrase “liver cells” might be used instead of the word hepatocytes”.

Although the phrase “in vitro” is commonly used in searches for replacement alternative methods, an exhaustive search may need additional use of terms such as “cell culture”, “tissue culture”, “organ culture”, “subcellular fractions”, etc.

Certain options available in some search systems can simplify some of the above considerations. These include:


Symbols such as * may be used in some search systems as wildcards to signify one or more characters. This can be a useful way of including different ways of spelling a word without having to input both versions, e.g.:
sul*ur will retrieve both sulfur and sulphur while truncation of the term: sulph* will retrieve sulphuric, sulphurous, sulphate, sulphite, etc, but not sulfuric, sulfurous, sulfate, sulfite, etc.
Note: it is not possible to combine a spelling wildcard with a truncation one.


Some of the pitfalls associated with the use of truncation operators are avoided in search engines which use stemming, i.e. which automatically search for terms grammatically related to the search terms. This may be confined to combining the singular and plural forms of common words. In other cases, stemming also applies to the combination of different grammatical forms, such as experiment, experimentation, experimental, experimenter, experimenting.

Boolean and other operators

The need to understand the use of operators may be questioned, given that database hosts and search engines now tend to provide a menu-driven search mode as well as an advanced interface in which operators may be used. However, menu-driven search interfaces may be limited in the search possibilities they offer.
Most online databases and Internet search engines permit the use of search operators. These operators define the relationship between the search terms being used. They act as commands to the search engine to specify how the sets of identified documents should be combined.

Further information: Searching for information Boolean operators

Further information: Searching for information Other operators