CENTRAL creation details

CENTRAL is created through the merging of relevant records retrieved from MEDLINE, Embase, Cochrane Review Groups' Specialised Registers and handsearching activities across the Cochrane Collaboration. It is compiled by Metaxis Ltd in the Cochrane Register of Studies and passed to John Wiley & Sons for publication. A detailed description of CENTRAL creation methods, past and present, is provided below.

Identifying MEDLINE RCTs and CCTs for inclusion in CENTRAL

Metaxis, on behalf of The Cochrane Collaboration, identify all the records in PubMed that are indexed with the Publication Type (PT) randomized controlled trial or controlled clinical trial (or both) in humans. To identify ‘human’ records, they identify all those records that are indexed as humans, humans and animals, or neither humans nor animals - and exclude only those records that are indexed as animals but not also humans. In addition to identifying ‘new’ records that are added to PubMed and meet the above criteria, they also replace any records that have undergone any corrections / updates (such as corrections to the citation, updating of indexing, links to retractions / errata etc.).
The PubMed search string used by Metaxis to generate the record set is:


(("randomized controlled trial"[Publication Type]) OR ("controlled clinical trial"[Publication Type])) NOT (ANIMALS[MH] NOT HUMANS[MH]) AND (“startdate"[LR] : "enddate"[LR] OR "startdate"[CRDT] : "enddate"[CRDT])

To reflect changes in healthcare practice and to conform with terminology in current use, the MEDLINE thesaurus (MeSH) is updated annually by the US National Library of Medicine, with new terms added, some old terms removed and some terms replaced. At the end of each year, when the new MeSH thesaurus is released, it is added to The Cochrane Library’s search functionality, to synchronize with the updating of the MeSH terms assigned to the MEDLINE records (usually in December each year).

A substantial proportion of the MEDLINE records indexed as randomized controlled trial or controlled clinical trial in the Publication Type field have been indexed as such as a result of the work of The Cochrane Collaboration (Lefebvre et al 2011). Handsearch results from Cochrane entities have been sent to the US National Library of Medicine, where MEDLINE records have been re-tagged with the publication types randomized controlled trial or controlled clinical trial as appropriate. In addition, the US Cochrane Center (USCC) and the UK Cochrane Centre (UKCC) conducted a search of MEDLINE for randomized controlled trials and quasi-randomized controlled trials using the Cochrane Highly Sensitive Search Strategy for the following years: US Cochrane Center (1966–1984; 1998–2004) and the UK Cochrane Centre (1985–1997) (Lefebvre et al 2011). All of the trials identified through this search are also included in CENTRAL. This project to re-tag randomized controlled trials and quasi-randomized controlled trials in MEDLINE ceased after the processing of the 2004 data was complete, due to lack of funding. MEDLINE records tagged with the randomized controlled trial or controlled clinical trial Publication Type continue, however, to be used as a core building block for the development of CENTRAL and the Cochrane Register of Studies (Identifying Embase RCTs and CCTs for inclusion in CENTRALCRS).


Identifying Embase RCTs and CCTs for inclusion in CENTRAL  

A retrospective search conducted by the UK Cochrane Centre (UKCC) for reports of trials in Embase has been completed for the years 1980 to 2009, for the following free-text terms: random$; factorial$; crossover$; placebo$; doubl$ adj blind$; singl$ adj blind$; assign$; allocat$; volunteer$; and for the following index terms, known as Emtree terms: crossover-procedure; double-blind procedure; randomized controlled trial; single-blind procedure. A search for the years 1974 to 1979 inclusive has also been completed for the free-text terms: random$; factorial$; crossover$ and placebo$. In addition, for the 2009 data set the following terms were searched limited to the title only: trial, comparison. For the 2009 data set the following terms were no longer searched: factorial$, assign$, volunteer$.

The UKCC continued to process Embase records until 2011; the last set being records published in 2010. For this 2010 Embase data set the following index terms were searched: crossover procedure, double-blind procedure, single-blind procedure and randomized controlled trial; the following free-text terms were searched limited to the title, abstract and original title fields only: crossover$, cross over$, placebo$, doubl$ adj blind$, allocat$, random$; and limited to the title only: trial$. For the first time the data set included a large number of conference abstracts newly added to Embase by Elsevier. The first results of the 2010 search, c. 1700 records, were added to CENTRAL in October 2011 (Issue 4) and additional c. 3100 records were submitted for publication in January 2012.

In March 2013, the contract to identify Embase records was awarded to a consortium made up of Metaxis Ltd, the Cochrane Dementia and Cognitive Improvement Group, and York Health Economics Consortium (YHEC).   A search covering January 2011 to December 2013 was run, from which 28,442 unique Embase records were identified and published in CENTRAL, January 2014 (Issue 1). All these records were identified from a search in Embase (via Ovid SP) using the Emtree headings Randomized Controlled Trial (RCT) or Controlled Clinical Trial (CCT). It is estimated that this search, using only these two headings, identified 2/3 of records eligible for inclusion in CENTRAL from the 2011-2013 backlog. Work to develop a search strategy to identify the remaining third, (i.e. records not indexed with the RCT or CCT term), is ongoing. 

The backlog of records without RCT/CCT headings is currently being screened and will be loaded into CENTRAL as they become available. Records identified in prospective Embase searches will also be loaded on a monthly basis beginning in March 2014. Records with RCT/CCT tags are loaded into CENTRAL in the issue following the month they appeared in Embase. For example, records that appeared in Embase during February are retrieved in March and appear in the March issue Records that do not have RCT/CCT tags are retrieved and screened in the month after they appeared in Embase, before being uploaded to CENTRAL the month after that. For example, records that do not have RCT/CCT tags and appeared in Embase during February are retrieved and screened in March and will appear in the April issue. 

The records added in 2014 issue 1 did not include conference proceedings reports. The backlog of conference proceedings is also being processed and will be loaded to CENTRAL as they become available. The process of retrieving and screening those records started in March 2014. 

The timescale for clearing the backlog of all records (including conference abstracts) is the end of November 2014.

Specialised Registers

Each of the approximately 50 Review Groups within the Collaboration is responsible for the development of a Specialised Register. The Trials Search Co-ordinators of the Review Groups flag records in their Specialised Registers for inclusion in CENTRAL. All records flagged for inclusion in CENTRAL are taken by Wiley on a fixed date each month. If a Review Group does not add any CENTRAL records during the month the Group's existing records are published on CENTRAL for that issue. Each record that has been submitted as part of a specialised register is assigned a specialised register code in CENTRAL. (See Appendix for a list of specialised register codes.)

Handsearch Results

Each of the Cochrane Centres has the responsibility for searching the general healthcare literature of its country or region, and most have now designated one or more staff members to co-ordinate this effort. Handsearching is also conducted by some Cochrane Review Groups and Fields/Networks in the specialist literature in their areas of interest. Currently there are approximately 2,400 journals being handsearched. Identified trial reports that are not relevant to a Review Group's scope and thus are not appropriate for their specialised register are submitted to CENTRAL as handsearch records and are assigned the HS-HANDSEARCH code as well as being assigned the handsearch code from the Appendix list below. 

Building CENTRAL

Each month, CENTRAL is re-built, using records from the four sources mentioned above, in the following order of precedence: (1) MEDLINE, (2) Embase, (3) handsearch results and (4) Specialised Registers. Therefore, for example, if a Specialised Register record matches to an existing MEDLINE or Embase record, the MEDLINE or Embase source record will be preferentially published. In these cases, the relevant Specialised Register code will be appended to the MEDLINE or Embase record in the CENTRAL ‘Cochrane Group Code’ field. No other information from the record, as originally submitted through the Specialised Register, will be added to the corresponding MEDLINE or Embase record that is published in CENTRAL.
CENTRAL was created as an immediate repository for all citations to reports of trials identified by the Collaboration. Because of the required quick turn-around time and relative lack of quality control, CENTRAL inevitably contains some typographical errors, duplicates, and reports of non-trials. Thus it is designed primarily for Collaboration use, although as part of The Cochrane Library it is accessible to all users. As resources become available in the future, CENTRAL will be ‘cleaned’ to contain only those reports of studies which are eligible for inclusion in a Cochrane Review.


Lefebvre C, Manheimer E, Glanville J. Chapter 6: Searching for studies. In: Higgins JPT, Green S (editors). Cochrane Handbook for Systematic Reviews of Interventions. Version 5.1.0 [updated March 2011]. The Cochrane Collaboration, 2011. http://www.cochrane-handbook.org/

Appendix: Review Group or Field/Network Specialised Register and Handsearch Codes

SR code HS code
Group name
SR-ADDICTN HS-ADDICTN Drugs and Alcohol Group
SR-ARI HS-ARI Acute Respiratory Infections Group
SR-BEHAV HS-BEHAV Developmental, Psychosocial and Learning Problems Group
SR-BEHAVMED HS-BEHAVMED Behavioural Medicines Field
SR-CF HS-CF Cystic Fibrosis and Genetic Disorders Group
SR-CHILD HS-CHILD Child Health Field
SR-COLOCA HS-COLOCA Colorectal Cancer Group
SR-COMMUN HS-COMMUN Consumers and Communication Group
SR-COMPMED HS-COMPMED Complementary Medicine Field
SR-DEMENTIA HS-DEMENTIA Dementia and Cognitive Impairment Group
SR-DEPRESSN HS-DEPRESSN Depression, Anxiety and Neurosis Group
SR-ENDOC HS-ENDOC Metabolic and Endocrine Disorders Group
SR-ENT HS-ENT Ear, Nose and Throat Disorders Group
SR-EPOC HS-EPOC Effective Practice and Organisation of Care Group
SR-EYES HS-EYES Eyes and Vision Group
SR-FERTILREG HS-FERTILREG Fertility Regulation Group
SR-GYNAECA HS-GYNAECA Gynaecological Cancer Group
SR-HAEMATOL HS-HAEMATOL Haematological Malignancies Group
SR-HTN HS-HTN Hypertension Group
SR-IBD HS-IBD Inflammatory Bowel Disease Group
SR-INCONT HS-INCONT Incontinence Group
SR-INFECTN HS-INFECTN Infectious Diseases Group
SR-INJ HS-INJ Injuries Group
SR-LIVER HS-LIVER Hepato-Biliary Group
SR-MENSTR HS-MENSTR Menstrual Disorders and Subfertility Group
SR-MOVEMENT HS-MOVEMENT Movement Disorders Group
SR-MS HS-MS Multiple Sclerosis Group
SR-MUSKEL HS-MUSKEL Musculoskeletal Group
SR-MUSKINJ HS-MUSKINJ Bone, Joint and Muscle Trauma Group
SR-NEUROMUSC HS-NEUROMUSC Neuromuscular Disease Group
SR-ORAL HS-ORAL Oral Health Group
- HS-PRECENTRL Handsearch records lost from CENTRAL before 2000 issue 1
SR-PREG HS-PREG Pregnancy and Childbirth Group
SR-PROSTATE HS-PROSTATE Prostatic Diseases and Urologic Cancers Group
SR-PVD HS-PVD Peripheral Vascular Diseases Group



Public Health Group
SR-REHAB HS-REHAB Rehabilitation and Related Therapies Field
SR-SCHIZ HS-SCHIZ Schizophrenia Group
SR-SPECTR HS-SPECTR Social, Psychological, and Educational Controlled Trials Register
SR-STD HS-STD Sexually Transmitted Diseases Group
SR-SYMPT HS-SYMPT Pain, Palliation and Supportive Care Group
SR-TOBACCO HS-TOBACCO Tobacco Addiction Group
SR-UPPERGI HS-UPPERGI Upper Gastrointestinal and Pancreatic Diseases Group

Centre Handsearch codes

HS code Centre
HS-ACC Australasian Cochrane Centre
HS-BCC Brazilian Cochrane Centre
HS-CANCC Canadian Cochrane Centre
HS-CHINESECC Chinese Cochrane Centre
HS-DCC Dutch Cochrane Centre
HS-GCC German Cochrane Centre
HS-IBEROCC Iberoamerican Cochrane Centre
HS-ITALCC Italian Cochrane Centre
HS-NCC Nordic Cochrane Centre
HS-SACC South African Cochrane Centre
HS-SASIANCC South Asian Cochrane Centre
HS-TCN Thai Cochrane Centre
HS-UKCC UK Cochrane Centre
HS-USCC US Cochrane Centre
HS-NBSSI National Blood Service Systematic Review Initiative

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