CENTRAL creation details

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.

Retrieving MEDLINE RCTs and CCTs into 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 (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).

Identifying Embase RCTs and CCTs for inclusion in CENTRAL


A retrospective search for reports of trials in Embase was completed by the UK Cochrane Centre (UKCC) for the years 1980 to 2009. For 1980 to 2008 the free-text terms searched were: random$; factorial$; crossover$; placebo$; doubl$ adj blind$; singl$ adj blind$; assign$; allocat$; volunteer$; and the index terms (known as Emtree terms) searched were: crossover-procedure; double-blind procedure; randomized controlled trial; single-blind procedure. For 2009 the same searches were run but the terms trial and comparison were limited to the title only and the terms factorial$, assign$ and volunteer$ were no longer included.

A separate search for the years 1974 to 1979 inclusive was completed using the free-text terms: random$; factorial$; crossover$ and placebo$.

The UKCC continued to process Embase records until 2011; the last set comprised all records published in 2010. Records for this set were identified using the following index terms: crossover procedure, double-blind procedure, single-blind procedure and randomized controlled trial; and 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$. The term trial$ was searched limited to the title only. The 2010 data set was added to CENTRAL in two batches: the first batch of approximately 1,700 records was added to CENTRAL in October 2011 (Issue 4) and the second batch of approximately 3,100 records was added in January 2012 (Issue 1).

 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). That new contract covered both clearing the backlog of records that had not been added to CENTRAL between the end of the UKCC contract and the start of the new contract, and the ongoing addition of new Embase records. Some Embase records go directly into CENTRAL as described below and others need to be screened to determine whether they meet the criteria for inclusion in CENTRAL. A description of the screening process is provided below.

Clearing the backlog

A search of Embase covering January 2011 to December 2013, was run via Ovid SP using the Emtree headings Randomized Controlled Trial (RCT) or Controlled Clinical Trial (CCT), from which 28,442 unique Embase records were identified and published in CENTRAL in January 2014 (Issue 1). It is estimated that this search, using only these two headings, identified around two thirds of the eligible records from the 2011-2013 Embase backlog.

The same search was used to retrieve conference abstracts published between 2010 and October 2014 and resulted in the publication of 9193 records in CENTRAL in October 2014 (Issue 10).

A further 20,655 records were identified through a screening process covering January 2011 to December 2013, inclusive, for journal publications, and January 2010 to December 2013, inclusive, for conference records. These records had been identified through the UKCC search strategy and did not have the Emtree headings RCT or CCT. They were published in CENTRAL in December 2014 (Issue 12).

The info graphic below details the numbers retrieved and the numbers published for the backlog period for both journal records and for conference records

Backlog records that need to be screened

backlog records that were automatic

Ongoing search and retrieval of records

Since the beginning of March 2014 eligible Embase records have been identified prospectively. This process entails the identification of two sets of records each month: Set 1 is identified using the Emtree headings Randomized Controlled Trial (RCT) or Controlled Clinical Trial (CCT), and Set 2 is identified by a newly developed search outlined below.

Set 1: Records with Publication Type RCT/CCT

The first set of records retrieved each month, using the Randomized Controlled Trial (RCT) or Controlled Clinical Trial (CCT) Emtree headings, have the following filter applied to help identify animal studies:

  1. exp experimental organism/
  2. animal tissue/
  3. animal cell/
  4. exp animal disease/
  5. exp carnivore disease/
  6. exp bird/
  7. exp experimental animal welfare/
  8. exp animal husbandry/
  9. animal behavior/
  10. exp animal cell culture/
  11. exp mammalian disease/
  12. exp mammal/
  13. exp marine species/
  14. nonhuman/
  15. animal.hw.
  16. or/1-15
  17. 16 not human/

This animal filter is being refined and until a final version is agreed, the records identified as very likely animal studies have been fed into the screening process each month. This is important because it gives empirical data on whether a record should have been rejected by the filter or not and helps further animal filter development. We expect the animal filter to be completed in quarter 4, 2014, though we also expect it to be further refined as the project progresses. Part of the purpose of the Challenge was to clear as many records as possible, but a secondary aim was to provide a rich human-generated data set that can be used to refine the animal filter described below so that it can be applied with confidence to the backlog to reduce the number of records needed to screen.

Records with the publication type RCT or CCT 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 of CENTRAL.

Set 2: Records retrieved by the new search

The development of the search strategy used to retrieve the second set of records each month has evolved throughout 2014-15. Currently the following string, run since January 2015, used is:

  1. Randomized controlled trial/
  2. Controlled clinical study/
  3. 1 or 2
  4. Random$.ti,ab.
  5. randomization/
  6. intermethod comparison/
  7. placebo.ti,ab.
  8. (compare or compared or comparison).ti.
  9. ((evaluated or evaluate or evaluating or assessed or assess) and (compare or compared or comparing or comparison)).ab.
  10. (open adj label).ti,ab.
  11. ((double or single or doubly or singly) adj (blind or blinded or blindly)).ti,ab.
  12. double blind procedure/
  13. parallel group$1.ti,ab.
  14. (crossover or cross over).ti,ab.
  15. ((assign$ or match or matched or allocation) adj5 (alternate or group$1 or intervention$1 or patient$1 or subject$1 or participant$1)).ti,ab.
  16. (assigned  or allocated).ti,ab.
  17. (controlled adj7 (study or design or trial)).ti,ab.
  18. (volunteer or volunteers).ti,ab.
  19. human experiment/
  20. trial.ti.
  21. or/4-20
  22. 21 not 3
  23. random$ adj sampl$ adj7 ("cross section$" or questionnaire$1 or survey$ or database$1)).ti,ab. not (comparative study/ or controlled study/ or randomi?ed controlled.ti,ab. or randomly assigned.ti,ab.)
  24. Cross-sectional study/ not (randomized controlled trial/ or controlled clinical study/ or controlled study/ or randomi?ed controlled.ti,ab. or control group$1.ti,ab.)
  25. (((case adj control$) and random$) not randomi?ed controlled).ti,ab.
  26. (Systematic review not (trial or study)).ti.
  27. (nonrandom$ not random$).ti,ab.
  28. "Random field$".ti,ab.
  29. (random cluster adj3 sampl$).ti,ab.
  30. (review.ab. and review.pt.) not trial.ti.
  31. "we searched".ab. and (review.ti. or review.pt.)
  32. "update review".ab.
  33. (databases adj4 searched).ab.
  34. (rat or rats or mouse or mice or swine or porcine or murine or sheep or lambs or pigs or piglets or rabbit or rabbits or cat or cats or dog or dogs or cattle or bovine or monkey or monkeys or trout or marmoset$1).ti. and animal experiment/
  35. Animal experiment/ not (human experiment/ or human/)
  36. or/23-35
  37. 22 not 36

This set of records is retrieved and screened in the month after they appeared in Embase, before being added to CENTRAL the month after that. For example, records that do not have RCT/CCT headings and appeared in Embase during February are retrieved and screened in March and will appear in the April issue of CENTRAL.

Screening records

Records for which CENTRAL eligibility is unclear (i.e. records from Set 2) are presented to a pool of screeners using a Web-based screening tool. Every record is screened at least twice with all disagreements resolved by two experienced expert screeners.

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. 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. 

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, but 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 Cochrane library. Emerging Themes in Epidemiology 2008;5:13. http://www.ete-online.com/content/5/1/13

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

SR codeHS codeGroup 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-CHILDCA HS-CHILDCA Childhood Cancer Group
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 Social, Psychological, and Educational Controlled Trials Register
SR-TOBACCO HS-TOBACCO Tobacco Addiction Group
SR-UPPERGI HS-UPPERGI Upper Gastrointestinal and Pancreatic Diseases Group

Centre Handsearch codes

HS CodeCentre
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