Clinical Evidence Field Guide

Ovid Technologies Field Guide

Clinical Evidence

Scope

Ovid is now offering Clinical Evidence as an individual title for use by clinicians and other health professionals. Clinical Evidence is produced by the BMJ Publishing Group and is frequently updated. It provides a concise account of the current state of knowledge, ignorance, and uncertainty about the prevention and treatment of a wide range of clinical conditions based on thorough searches and appraisal of the literature. In other words, it contains a compendium of evidence on the effects of common clinical interventions. The complete text of this publication is available in a highly interlinked and easily navigated graphical interface, similar to Books@Ovid.

Highlights

How Clinical Evidence Is Put Together

The summaries in Clinical Evidence result from a rigorous process aimed at ensuring that the information they contain is both reliable and relevant to clinical practice.

Selecting Topics

Clinical Evidence aims to cover common or important clinical conditions seen in primary and hospital care. To decide which conditions to cover in the first and second issues, its producers reviewed national data on consultation rates, morbidity, and mortality, and took advice from generalist clinicians and patient groups.

Selecting the Questions

The questions in Clinical Evidence concern the benefits and harms of preventative and therapeutic interventions, with emphasis on outcomes that matter to patients. Questions are selected for their relevance to clinical practice by section advisors and contributors, in collaboration with primary care clinicians and patient groups. Each new issue of Clinical Evidence will include new questions as well as updates of existing questions. (Readers can suggest new clinical questions on the Clinical Evidence website, http://www.clinicalevidence.org, or by writing directly to Clinical Evidence.)

Searching and Appraising the Literature

For each question, the literature is searched using the Cochrane Library, MEDLINE, EMBASE, and, occasionally, other electronic databases. The producers first look for good systematic reviews and then for primary studies with designs that are most appropriate for the topic. For most questions on interventions, this means finding randomized controlled trials, but some questions, such as those examining aetiology, prognosis, or rare outcomes, may require observational studies. The date of the search is recorded in the Methods section for each topic.

Of the studies identified in the search, the producers of Clinical Evidence select and summarize only a small proportion. The selection is done by critically appraising the abstracts of the studies identified in the search, a task performed independently by two information scientists using validated criteria. Where the search identifies more than one or two good reviews or trials, the producers select those they judge to be the most robust or relevant, using the full text of the article. Where the producers identify few or no good reviews or trials, they include other studies but highlight their limitations. Contributors, who are chosen for their expertise in the field and their skills in epidemiology, are asked to review the selection of studies, and to justify any additions or exclusions they wish to make.

An overview of Clinical Evidence's search strategy and critical appraisal process is available on their web site (www.evidence.org).

Summarizing the Evidence, Peer Review, and Editing

The contributors summarize the evidence relating to each question. Each topic is then peer reviewed by the section advisors, and by at least three external expert clinicians. The revised text is then extensively edited by editors with clinical and epidemiological training, and data are checked against the original study reports.

Navigation

When users select Clinical Evidence from Ovid's database menu, they will find that its search environment is different from other Ovid databases. (For detailed searching information, see the online Help available during searches.) Single-frame or multiple-frame environments contain the retrieved text. Buttons and bars on the top and side of the screen provide navigational buttons, links to the search page, and access to Help. Search screens provide links to Home, Logoff, Help, and the main Ovid database screen. If the user is accessing a specific chapter, links are available to the text's Table of Contents and Index and to the Search and Results screens, making it easier for searchers to jump from one chapter to another.

Ovid and Clinical Evidence: A Feature Comparison

Although the Clinical Evidence database is selected from the database menu just like any other database available in Ovid Web Gateway, its search environment has a different look and feel than other Ovid databases. For an overview of the ways in which Clinical Evidence is different, check the following table.

Function Ovid Clinical Evidence

Default Boolean Operator ADJ (meaning Adjacency); creates a more focused search set. OR; creates a broader search set. When using Clinical Evidence's Exact Match feature, OR becomes ADJ.

Stopwords
Stopwords are terms that are not indexed, and are ignored when encountered in a search string. For example, if "of" were a stopword, then "quality of life" would be searched as "quality adj life".
Ovid has approximately 156 stopwords. Most are also used as stopwords in Clinical Evidence. Ovid does not index terms that are stopwords, and ignores them in a search string. Clinical Evidence has 216 stopwords. Clinical Evidence does not index terms that are stopwords, and ignores them in a search string.

Search Stemming There is no equivalent function in Ovid. Stemming in Clinical Evidence is used to search for variants of a search term, and is enabled as the default. For example, the query term bleed retrieves bleed, bleeds, and bleeding. Searchers can disable stemming by using wild card characters that force the search engine to search exact matches of the query term.

Search History Results of the user's search history are maintained on the Main Search page and are exploded by using the Expand Search tab. Search histories can also be saved temporarily (for 24 hours) or permanently. Only the current search can be retrieved, by clicking on the Results button.

Exact Search Match String Quotation marks force the search engine to search terms exactly as entered (minus stopwords). Instead of quotation marks, Clinical Evidence uses the Match Query Phrase function. In the interface, searchers can check the "Exact Match Only" box to have an exact search match. On the command line, searchers can use single quotation marks around search term strings to produce the same results.

Default Search Fields Database specific. In Clinical Evidence the default search fields are title, text, author, table, figure captions, and references. This corresponds to the All Fields Checked option.

Default Display Fields Database specific. However, most displays contain title, author, and source fields. Product name, document name, and relevance score (expressed as a percentage). Hits are listed in order of relevancy ranking.
Thesaurus Enabled by default for all databases that support this function. Searchers use the command "thes" or use Tools to enter a term and retrieve broader, narrower, or related terms. Not available in Clinical Evidence.

Results Display Order Displays in chronological order, most recent first. Uses relevance ranking. This is based on:

Breadth of Match: higher scores are given to documents containing more distinct query terms.
Inverse Document Frequency: Higher scores are given to documents that contain query terms occurring rarely in the database as a whole.
Frequency: higher scores are given to documents that contain multiple occurrences of individual query terms.

Find Similar Not implemented in Ovid. The Query by Example feature uses the top relevance-ranked terms from the document as the query, performing a search to find similar documents.

Both Ovid and Clinical Evidence use Boolean operators. Again, however, the way they use them differs significantly in some ways.

Boolean Operator Ovid Clinical Evidence

AND Retrieves citations only if they contain all query terms. The same.

OR Retrieves all citations containing either of the query terms, with duplicates eliminated. Clinical Evidence's default setting. Retrieves all citations containing either of the query terms.

NOT Retrieves documents that contain the first query term but specifically exclude the second query term. Example: health reform NOT health maintenance organizations. The same.

ADJ Ovid's default setting. Assumes adjacency and inserts the ADJ operator. Query terms are retrieved in a specific order, excluding stopwords. No equivalent. However, the Match Exact Query assumes adjacency.

FREQ Specifies a threshold of occurrence. For example, "blood pressure.tx/freq=10" retrieves only documents in which the phrase "blood pressure" occurs 10 or more times in the specified field (in this example, full text). No equivalent.

Function	Ovid	Clinical Evidence
Default Boolean Operator	ADJ (meaning Adjacency); creates a more focused search set.	OR; creates a broader search set. When using Clinical Evidence's Exact Match feature, OR becomes ADJ.
Stopwords Stopwords are terms that are not indexed, and are ignored when encountered in a search string. For example, if "of" were a stopword, then "quality of life" would be searched as "quality adj life".	Ovid has approximately 156 stopwords. Most are also used as stopwords in Clinical Evidence. Ovid does not index terms that are stopwords, and ignores them in a search string.	Clinical Evidence has 216 stopwords. Clinical Evidence does not index terms that are stopwords, and ignores them in a search string.
Search Stemming	There is no equivalent function in Ovid.	Stemming in Clinical Evidence is used to search for variants of a search term, and is enabled as the default. For example, the query term bleed retrieves bleed, bleeds, and bleeding. Searchers can disable stemming by using wild card characters that force the search engine to search exact matches of the query term.
Search History	Results of the user's search history are maintained on the Main Search page and are exploded by using the Expand Search tab. Search histories can also be saved temporarily (for 24 hours) or permanently.	Only the current search can be retrieved, by clicking on the Results button.
Exact Search Match String	Quotation marks force the search engine to search terms exactly as entered (minus stopwords).	Instead of quotation marks, Clinical Evidence uses the Match Query Phrase function. In the interface, searchers can check the "Exact Match Only" box to have an exact search match. On the command line, searchers can use single quotation marks around search term strings to produce the same results.
Default Search Fields	Database specific.	In Clinical Evidence the default search fields are title, text, author, table, figure captions, and references. This corresponds to the All Fields Checked option.
Default Display Fields	Database specific. However, most displays contain title, author, and source fields.	Product name, document name, and relevance score (expressed as a percentage). Hits are listed in order of relevancy ranking.
Thesaurus	Enabled by default for all databases that support this function. Searchers use the command "thes" or use Tools to enter a term and retrieve broader, narrower, or related terms.	Not available in Clinical Evidence.
Results Display Order	Displays in chronological order, most recent first.	Uses relevance ranking. This is based on: Breadth of Match: higher scores are given to documents containing more distinct query terms. Inverse Document Frequency: Higher scores are given to documents that contain query terms occurring rarely in the database as a whole. Frequency: higher scores are given to documents that contain multiple occurrences of individual query terms.
Find Similar	Not implemented in Ovid.	The Query by Example feature uses the top relevance-ranked terms from the document as the query, performing a search to find similar documents.

Boolean Operator	Ovid	Clinical Evidence
AND	Retrieves citations only if they contain all query terms.	The same.
OR	Retrieves all citations containing either of the query terms, with duplicates eliminated.	Clinical Evidence's default setting. Retrieves all citations containing either of the query terms.
NOT	Retrieves documents that contain the first query term but specifically exclude the second query term. Example: health reform NOT health maintenance organizations.	The same.
ADJ	Ovid's default setting. Assumes adjacency and inserts the ADJ operator. Query terms are retrieved in a specific order, excluding stopwords.	No equivalent. However, the Match Exact Query assumes adjacency.
FREQ	Specifies a threshold of occurrence. For example, "blood pressure.tx/freq=10" retrieves only documents in which the phrase "blood pressure" occurs 10 or more times in the specified field (in this example, full text).	No equivalent.