CENTRAL is comprised of a merge of relevant records retrieved from MEDLINE, relevant records retrieved from EMBASE, all Review Groups' Specialised Registers and the handsearch results register, as described below.
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 (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 (CRS).
A retrospective search conducted by the UK Cochrane Centre 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$, volunteer$.
For the 2010 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). In January 2012 an additional c. 3100 records were submitted for publication and the remainder will be added to CENTRAL in subsequent issues.
The searches across all years of this project have yielded a total of c. 105,000 reports of trials not indexed, at the time of the search, as randomized controlled trial or controlled clinical trial in MEDLINE. All of these reports are now published in CENTRAL (Lefebvre et al 2008).
This project to identify reports of trials in Embase will no longer be conducted by the UK Cochrane, after the work on the 2010 dataset is complete. Other mechanisms for continuing this work thereafter are currently being investigated by the Cochrane Editorial Unit and others.
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.)
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. The Cochrane Review Groups and Fields/Networks are responsible for co-ordinating searching of 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. These handsearch results must be prepared according to specifications as published in the ‘Guide for Submission of Handsearch Results to CENTRAL’ in the CENTRAL Management Plan.
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/
Lefebvre C, Eisinga A, McDonald S, Paul N. Enhancing access to reports of clinical trials published world-wide – the contribution of EMBASE records to the Cochrane Central Register of Controlled Trials (CENTRAL) in The Cochrane Library. Emerging Themes in Epidemiology 2008;5:13. http://www.ete-online.com/content/5/1/13
|SR code||HS code
|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-BREASTCA||HS-BREASTCA||Breast Cancer Group|
|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-IBD||HS-IBD||Inflammatory Bowel Disease Group|
|SR-INFECTN||HS-INFECTN||Infectious Diseases Group|
|SR-LUNGCA||HS-LUNGCA||Lung Cancer 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-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-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|
|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|