The Strengthening the Reporting of Observational Studies in Epidemiology [STROBE] statement: guidelines for reporting observational studies

TL;DR: The Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) initiative developed recommendations on what should be included in an accurate and complete report of an observational study, resulting in a checklist of 22 items (the STROBE statement) that relate to the title, abstract, introduction, methods, results, and discussion sections of articles.

Abstract: Much biomedical research is observational. The reporting of such research is often inadequate, which hampers the assessment of its strengths and weaknesses and of a study's generalisability. The Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) Initiative developed recommendations on what should be included in an accurate and complete report of an observational study. We defined the scope of the recommendations to cover three main study designs: cohort, case-control, and cross-sectional studies. We convened a 2-day workshop in September 2004, with methodologists, researchers, and journal editors to draft a checklist of items. This list was subsequently revised during several meetings of the coordinating group and in e-mail discussions with the larger group of STROBE contributors, taking into account empirical evidence and methodological considerations. The workshop and the subsequent iterative process of consultation and revision resulted in a checklist of 22 items (the STROBE Statement) that relate to the title, abstract, introduction, methods, results, and discussion sections of articles. 18 items are common to all three study designs and four are specific for cohort, case-control, or cross-sectional studies. A detailed Explanation and Elaboration document is published separately and is freely available on the Web sites of PLoS Medicine, Annals of Internal Medicine, and Epidemiology. We hope that the STROBE Statement will contribute to improving the quality of reporting of observational studies.

Full text

ORIGINAL ARTICLES
The Strengthening the Reporting of Observational Studies in
Epidemiology (STROBE) statement: guidelines for reporting
observational studies
Erik von Elm
a,g
, Douglas G. Altman
c
, Matthias Egger
a,b,
*
, Stuart J. Pocock
d
,
Peter C. Gøtzsche
e
, Jan P. Vandenbroucke
f
for the STROBE Initiative
a
Institute of Social and Preventive Medicine (ISPM), University of Bern, 3012 Bern, Switzerland
b
Department of Social Medicine, University of Bristol, Bristol, United Kingdom
c
Centre for Statistics in Medicine, University of Oxford, Oxford, United Kingdom
d
London School of Hygiene and Tropical Medicine, University of London, London, United Kingdom
e
Nordic Cochrane Centre, Copenhagen, Denmark
f
Department of Clinical Epidemiology, Leiden University Hospital, Leiden, The Netherlands
g
Department of Medical Biometry and Medical Informatics, University Medical Centre, Freiburg, Germany
Accepted 18 November 2007
Abstract
Much of biomedical research is observational. The reporting of such research is often inadequate, which hampers the assessment of its
strengths and weaknesses and of a study’s generalizability. The Strengthening the Reporting of Observational Studies in Epidemiology
(STROBE) Initiative developed recommendations on what should be included in an accurate and complete report of an observational study.
We defined the scope of the recommendations to cover three main study designs: cohort, caseecontrol, and cross-sectional studies. We
convened a 2-day workshop in September 2004, with methodologists, researchers, and journal editors to draft a checklist of items. This
list was subsequently revised during several meetings of the coordinating group and in e-mail discussions with the larger group of STROBE
contributors, taking into account empirical evidence and methodological considerations. The workshop and the subsequent iterative process
of consultation and revision resulted in a checklist of 22 items (the STROBE Statement) that relate to the title, abstract, introduction,
methods, results, and discussion sections of articles. Eighteen items are common to all three study designs and four are specific for cohort,
caseecontrol, or cross-sectional studies. A detailed Explanation and Elaboration document is published separately and is freely available on
the web sites of PLoS Medicine, Annals of Internal Medicine, and Epidemiology. We hope that the STROBE Statement will contribute to
improving the quality of reporting of observational studies. Ó 2007 The authors. Published by Elsevier Inc. All rights reserved.
1. Introduction
Many questions in medical research are investigated in
observational studies [1]. Much of the research into the
cause of diseases relies on cohort, caseecontrol, or cross-
sectional studies. Observational studies also have a role in
research into the benefits and harms of medical interven-
tions [2]. Randomized trials cannot answer all important
questions about a given intervention. For example, observa-
tional studies are more suitable to detect rare or late adverse
effects of treatments and are more likely to provide an
indication of what is achieved in daily medical practice [3].
Research should be reported transparently so that
readers can follow what was planned, what was done, what
was found, and what conclusions were drawn. The credibil-
ity of research depends on a critical assessment by others of
the strengths and weaknesses in study design, conduct, and
analysis. Transparent reporting is also needed to judge
whether and how results can be included in systematic
reviews [4,5]. However, in published observational research
important information is often missing or unclear. An anal-
ysis of epidemiological studies published in general medi-
cal and specialist journals found that the rationale behind
the choice of potential confounding variables was often
* Corresponding author.
E-mail address: strobe@ispm.unibe.ch (M. Egger).
In order to encourage dissemination of the STROBE Statement, this ar-
ticle is freely accessible on the Journal of Clinical Epidemiology website
(http://www.jclinepi.com), and will also be published in Annals of Internal
Medicine, BMJ, Bulletin of the World Health Organization, Epidemiology,
The Lancet, PLoS Medicine, and Preventive Medicine. The authors jointly
hold the copyright of this article. For details on further use, see STROBE
website (http://www.strobe-statement.org).
0895-4356/08/$ e see front matter Ó 2007 The authors. Published by Elsevier Inc. All rights reserved.
doi: 10.1016/j.jclinepi.2007.11.008
Journal of Clinical Epidemiology 61 (2008) 344e349

not reported [6]. Only few reports of caseecontrol studies
in psychiatry explained the methods used to identify cases
and controls [7]. In a survey of longitudinal studies in
stroke research, 17 of 49 articles (35%) did not specify
the eligibility criteria [8]. Others have argued that without
sufficient clarity of reporting, the benefits of research might
be achieved more slowly [9], and that there is a need for
guidance in reporting observational studies [10,11].
Recommendations on the reporting of research can im-
prove reporting quality. The Consolidated Standards of
Reporting Trials (CONSORT) Statement was developed
in 1996 and revised 5 years later [12]. Many medical jour-
nals supported this initiative [13], which has helped to im-
prove the quality of reports of randomized trials [14,15].
Similar initiatives have followed for other research areasd
e.g., for the reporting of meta-analyses of randomized trials
[16] or diagnostic studies [17]. We established a network of
methodologists, researchers, and journal editors to develop
recommendations for the reporting of observational research:
the Strengthening the Reporting of Observational Studies in
Epidemiology (STROBE) Statement.
1.1. Aims and use of the STROBE Statement
The STROBE Statement is a checklist of items that should
be addressed in articles reporting on the three main study de-
signs of analytical epidemiology: cohort, caseecontrol, and
cross-sectional studies. The intention is solely to provide
guidance on how to report observational research well: these
recommendations are not prescriptions for designing or con-
ducting studies. Also, while clarity of reporting is a prerequi-
site to evaluation, the checklist is not an instrument to
evaluate the quality of observational research.
Here, we present the STROBE Statement and explain
how it was developed. In a detailed companion paper, the
Explanation and Elaboration article [18e20], we justify
the inclusion of the different checklist items and give meth-
odological background and published examples of what we
consider transparent reporting. We strongly recommend
using the STROBE checklist in conjunction with the
explanatory article, which is available freely on the web
sites of PLoS Medicine (http://www.plosmedicine.org/),
Annals of Internal Medicine (http://www.annals.org/), and
Epidemiology (http://www.epidem.com/).
1.2. Development of the STROBE Statement
We established the STROBE Initiative in 2004, obtained
funding for a workshop and set up a web site (http://
www.strobe-statement.org/). We searched textbooks, bib-
liographic databases, reference lists, and personal files for
relevant material, including previous recommendations,
empirical studies of reporting and articles describing rele-
vant methodological research. Because observational re-
search makes use of many different study designs, we felt
that the scope of STROBE had to be clearly defined early
on. We decided to focus on the three study designs that
are used most widely in analytical observational research:
cohort, caseecontrol, and cross-sectional studies.
We organized a 2-day workshop in Bristol, UK, in
September 2004. Twenty-three individuals attended this
meeting, including editorial staff from Annals of Internal
Medicine, BMJ, Bulletin of the World Health Organization,
International Journal of Epidemiology, JAMA, Preventive
Medicine, and The Lancet, as well as epidemiologists, meth-
odologists, statisticians, and practitioners from Europe and
North America. Written contributions were sought from
10 other individuals who declared an interest in contributing
to STROBE, but could not attend. Three working groups
identified items deemed to be important to include in check-
lists for each type of study. A provisional list of items pre-
pared in advance (available from our web site) was used
to facilitate discussions. The three draft checklists were then
discussed by all participants and, where possible, items were
revised to make them applicable to all three study designs.
In a final plenary session, the group decided on the strategy
for finalizing and disseminating the STROBE Statement.
After the workshop, we drafted a combined checklist in-
cluding all three designs and made it available on our web
site. We invited participants and additional scientists and
editors to comment on this draft checklist. We subsequently
published three revisions on the web site and two summa-
ries of comments received and changes made. During this
process the coordinating group (i.e., the authors of the pres-
ent paper) met on eight occasions for 1 or 2 days and held
several telephone conferences to revise the checklist and to
prepare the present paper and the Explanation and Elabora-
tion paper [18e20]. The coordinating group invited three
additional coauthors with methodological and editorial
expertise to help write the Explanation and Elaboration
paper, and sought feedback from more than 30 people,
who are listed at the end of this paper. We allowed several
weeks for comments on subsequent drafts of the paper and
reminded collaborators about deadlines by e-mail.
1.3. STROBE components
The STROBE Statement is a checklist of 22 items that
we consider essential for good reporting of observational
studies (Table 1). These items relate to the article’s title
and abstract (item 1), the introduction (items 2 and 3),
methods (items 4e12), results (items 13e17) and discus-
sion sections (items 18e21), and other information (item
22 on funding). Eighteen items are common to all three
designs, whereas four (items 6, 12, 14, and 15) are design
specific, with different versions for all or part of the item.
For some items (indicated by asterisks), information should
be given separately for cases and controls in caseecontrol
studies, or exposed and unexposed groups in cohort and
cross-sectional studies. Although presented here as a single
checklist, separate checklists are available for each of the
three study designs on the STROBE web site.
345E. von Elm et al. / Journal of Clinical Epidemiology 61 (2008) 344e349

Table 1
The STROBE statementdchecklist of items that should be addressed in reports of observational studies
Item number Recommendation
Title and abstract 1 (a) Indicate the study’s design with a commonly used term in the title or the abstract
(b) Provide in the abstract, an informative and balanced summary of what was done and what
was found
Introduction
Background/rationale 2 Explain the scientific background and rationale for the investigation being reported
Objectives 3 State specific objectives, including any prespecified hypotheses
Methods
Study design 4 Present key elements of study design early in the paper
Setting 5 Describe the setting, locations, and relevant dates, including periods of recruitment, exposure, follow
up, and data collection
Participants 6 (a) Cohort studydGive the eligibility criteria, and the sources and methods of selection of
participants. Describe methods of follow up
Case-control studydGive the eligibility criteria, and the sources and methods of case ascertainment
and control selection. Give the rationale for the choice of cases and controls
Cross-sectional studydGive the eligibility criteria, and the sources and methods of selection
of participants
(b) Cohort studydFor matched studies, give matching criteria and number of exposed and unexposed
Case-control studydFor matched studies, give matching criteria and the number of controls
per case
Variables 7 Clearly define all outcomes, exposures, predictors, potential confounders, and effect modifiers.
Give diagnostic criteria, if applicable
Data sources/measurement 8
*
For each variable of interest, give sources of data and details of methods of assessment
(measurement). Describe comparability of assessment methods if there is more than one group
Bias 9 Describe any efforts to address potential sources of bias
Study size 10 Explain how the study size was arrived at
Quantitative variables 11 Explain how quantitative variables were handled in the analyses. If applicable, describe which
groupings were chosen, and why
Statistical methods 12 (a) Describe all statistical methods, including those used to control for confounding
(b) Describe any methods used to examine subgroups and interactions
(c) Explain how missing data were addressed
(d) Cohort studydIf applicable, explain how loss to follow up was addressed
Case-control studydIf applicable, explain how matching of cases and controls was addressed
Cross-sectional studydIf applicable, describe analytical methods taking account of sampling
strategy
(e) Describe any sensitivity analyses
Results
Participants 13
*
(a) Report the numbers of individuals at each stage of the studyde.g., numbers potentially eligible,
examined for eligibility, confirmed eligible, included in the study, completing follow up,
and analyzed
(b) Give reasons for nonparticipation at each stage
(c) Consider use of a flow diagram
Descriptive data 14
*
(a) Give characteristics of study participants (e.g., demographic, clinical, social) and information
on exposures and potential confounders
(b) Indicate the number of participants with missing data for each variable of interest
(c) Cohort studydSummarize follow-up time (e.g., average and total amount)
Outcome data 15
*
Cohort studydReport numbers of outcome events or summary measures over time
Case-control studydReport numbers in each exposure category, or summary measures of exposure
Cross-sectional studydReport numbers of outcome events or summary measures
Main results 16 (a) Give unadjusted estimates and, if applicable, confounder-adjusted estimates and their precision
(e.g., 95% confidence interval). Make clear which confounders were adjusted for and why they
were included
(b) Report category boundaries when continuous variables were categorized
(c) If relevant, consider translating estimates of relative risk into absolute risk for a meaningful
time period
Other analyses 17 Report other analyses donede.g., analyses of subgroups and interactions, and sensitivity analyses
Discussion
Key results 18 Summarize key results with reference to study objectives
Limitations 19 Discuss limitations of the study, taking into account sources of potential bias or imprecision.
Discuss both direction and magnitude of any potential bias
(Continued )
346 E. von Elm et al. / Journal of Clinical Epidemiology 61 (2008) 344e349

1.4. Implications and limitations
The STROBE Statement was developed to assist authors
when writing up analytical observational studies, to support
editors and reviewers when considering such articles for
publication, and to help readers when critically appraising
published articles. We developed the checklist through an
open process, taking into account the experience gained
with previous initiatives, in particular CONSORT. We
reviewed the relevant empirical evidence as well as meth-
odological work and subjected consecutive drafts to an
extensive iterative process of consultation. The checklist
presented here is thus based on input from a large number
of individuals with diverse backgrounds and perspectives.
The comprehensive explanatory article [18e20], which is
intended for use alongside the checklist, also benefited
greatly from this consultation process.
Observational studies serve a wide range of purposes, on
a continuum from the discovery of new findings to the con-
firmation or refutation of previous findings [18e20].Some
studies are essentially exploratory and raise interesting
hypotheses. Others pursue clearly defined hypotheses in
available data. In yet another type of studies, the collection
of new data is planned carefully on the basis of an existing
hypothesis. We believe the present checklist can be useful
for all these studies, since the readers always need to know
what was planned (and what was not), what was done, what
was found, and what the results mean. We acknowledge
that STROBE is currently limited to three main observa-
tional study designs. We would welcome extensions that
adapt the checklist to other designsde.g., case-crossover
studies or ecological studiesd and also to specific topic
areas. Four extensions are now available for the CONSORT
statement [21e24]. A first extension to STROBE is under-
way for geneedisease association studies: the STROBE
Extension to Genetic Association studies (STREGA) initia-
tive [25]. We ask those who aim to develop extensions of
the STROBE Statement to contact the coordinating group
first to avoid duplication of effort.
The STROBE Statement should not be interpreted as an
attempt to prescribe the reporting of observational research
in a rigid format. The checklist items should be addressed
in sufficient detail and with clarity somewhere in an article,
but the order and format for presenting information
depends on author preferences, journal style, and the tradi-
tions of the research field. For instance, we discuss the re-
porting of results under a number of separate items, while
recognizing that authors might address several items within
a single section of text or in a table. Also, item 22, on the
source of funding and the role of funders, could be ad-
dressed in an appendix or in the methods section of the ar-
ticle. We do not aim at standardizing reporting. Authors of
randomized clinical trials were asked by an editor of a spe-
cialist medical journal to ‘‘CONSORT’’ their manuscripts
on submission [26]. We believe that manuscripts should
not be ‘‘STROBEd,’’ in the sense of regulating style or ter-
minology. We encourage authors to use narrative elements,
including the description of illustrative cases, to comple-
ment the essential information about their study, and to
make their articles an interesting read [27].
We emphasize that the STROBE Statement was not de-
veloped as a tool for assessing the quality of published ob-
servational research. Such instruments have been developed
by other groups and were the subject of a recent systematic
review [28]. In the Explanation and Elaboration paper, we
used several examples of good reporting from studies
whose results were not confirmed in further researchdthe
important feature was the good reporting, not whether the
research was of good quality. However, if STROBE is
adopted by authors and journals, issues such as confound-
ing, bias, and generalizability could become more transpar-
ent, which might help temper the overenthusiastic reporting
of new findings in the scientific community and popular
media [29], and improve the methodology of studies in
the long term. Better reporting may also help to have more
informed decisions about when new studies are needed, and
what they should address.
We did not undertake a comprehensive systematic
review for each of the checklist items and subitems, or do
our own research to fill gaps in the evidence base. Further,
although no one was excluded from the process, the
Table 1
Continued
Item number Recommendation
Interpretation 20 Give a cautious overall interpretation of results considering objectives, limitations, multiplicity of
analyses, results from similar studies, and other relevant evidence
Generalizability 21 Discuss the generalizability (external validity) of the study results
Other information
Funding 22 Give the source of funding and the role of the funders for the present study and, if applicable, for the
original study on which the present article is based
* Give such information separately for cases and controls in case-control studies, and, if applicable, for exposed and unexposed groups in cohort and
cross-sectional studies.
Note: An Explanation and Elaboration article discusses each checklist item and gives methodological background and published examples of transparent
reporting. The STROBE checklist is best used in conjunction with this article (freely available on the websites of PLoS Medicine at http://www.plosmedi
cine.org, Annals of Internal Medicine at http:/www.annals.org, and Epidemiology at http://www.epidem.com). Separate versions of the checklist for cohort,
case-control, and cross-sectional studies are available on the STROBE website at http://www.strobe-statement.org.
347E. von Elm et al. / Journal of Clinical Epidemiology 61 (2008) 344e349

composition of the group of contributors was influenced by
existing networks and was not representative in terms of ge-
ography (it was dominated by contributors from Europe
and North America) and probably was not representative
in terms of research interests and disciplines. We stress that
STROBE and other recommendations on the reporting of
research should be seen as evolving documents that require
continual assessment, refinement, and, if necessary, change.
We welcome suggestions for the further dissemination of
STROBEde.g., by republication of the present article in
specialist journals and in journals published in other
languages. Groups or individuals who intend to translate
the checklist to other languages should consult the coordi-
nating group beforehand. We will revise the checklist in the
future, taking into account comments, criticism, new
evidence, and experience from its use. We invite readers
to submit their comments via the STROBE web site
(http://www.strobe-statement.org/).
Acknowledgments
We are grateful to Gerd Antes, Kay Dickersin, Shah
Ebrahim, and Richard Lilford for supporting the STROBE
Initiative. We are grateful to the following institutions that
have hosted working meetings of the coordinating group:
Institute of Social and Preventive Medicine (ISPM),
University of Bern, Bern, Switzerland; Department of So-
cial Medicine, University of Bristol, Bristol, UK; London
School of Hygiene and Tropical Medicine, London, UK;
Nordic Cochrane Centre, Copenhagen, Denmark; and
Centre for Statistics in Medicine, Oxford, UK. We are
grateful to six reviewers who provided helpful comments
on a previous draft of this paper.
The following individuals have contributed to the
content and elaboration of the STROBE Statement: Doug-
las G Altman, Maria Blettner, Paolo Boffetta, Hermann
Brenner, Genevie
`
ve Che
ˆ
ne, Cyrus Cooper, George Davey-
Smith, Erik von Elm, Matthias Egger, France Gagnon,
Peter C Gøtzsche, Philip Greenland, Sander Greenland,
Claire Infante-Rivard, John Ioannidis, Astrid James, Giselle
Jones, Bruno Ledergerber, Julian Little, Margaret May,
David Moher, Hooman Momen, Alfredo Morabia, Hal
Morgenstern, Cynthia D Mulrow, Fred Paccaud, Stuart J
Pocock, Charles Poole, Drummond Rennie, Martin Ro
¨
o
¨
sli,
Dietrich Rothenbacher, Kenneth Rothman, Caroline Sabin,
Willi Sauerbrei, Lale Say, James J Schlesselman, Jonathan
Sterne, Holly Syddall, Jan P Vandenbroucke, Ian White,
Susan Wieland, Hywel Williams, Guang Yong Zou.
The workshop was funded by the European Science
Foundation (ESF). Additional funding was received from
the Medical Research Council Health Services Research
Collaboration and the National Health Services Research
and Development Methodology Programme. The funders
had no role in the elaboration of the STROBE Statement,
decision to publish it, or preparation of the manuscript.
The authors have declared that no competing interests
exist.
References
[1] Glasziou P, Vandenbroucke JP, Chalmers I. Assessing the quality of
research. BMJ 2004;328:39e41.
[2] Black N. Why we need observational studies to evaluate the effec-
tiveness of health care. BMJ 1996;312:1215e8.
[3] Papanikolaou PN, Christidi GD, Ioannidis JP. Comparison of
evidence on harms of medical interventions in randomized and
nonrandomized studies. CMAJ 2006;174:635e41.
[4] Ju
¨
ni P, Altman DG, Egger M. Systematic reviews in health care: As-
sessing the quality of controlled clinical trials. BMJ 2001;323:42e6.
[5] Egger M, Schneider M, Davey Smith G. Spurious precision?
Meta-analysis of observational studies. BMJ 1998;316:140e4.
[6] Pocock SJ, Collier TJ, Dandreo KJ, de Stavola BL, Goldman MB,
Kalish LA, et al. Issues in the reporting of epidemiological studies:
a survey of recent practice. BMJ 2004;329:883.
[7] Lee W, Bindman J, Ford T, Glozier N, Moran P, Stewart R, et al. Bias
in psychiatric case-control studies: literature survey. Br J Psychiatry
2007;190:204e9.
[8] Tooth L, Ware R, Bain C, Purdie DM, Dobson A. Quality of reporting
of observational longitudinal research. Am J Epidemiol 2005;161:
280e8.
[9] Bogardus ST Jr, Concato J, Feinstein AR. Clinical epidemiological
quality in molecular genetic research: the need for methodological
standards. JAMA 1999;281:1919e26.
[10] Anonymous. Guidelines for documentation of epidemiologic studies.
Epidemiology Work Group of the Interagency Regulatory Liaison
Group. Am J Epidemiol 1981;114:609e13.
[11] Rennie D. CONSORT reviseddimproving the reporting of random-
ized trials. JAMA 2001;285:2006e7.
[12] Moher D, Schulz KF, Altman DG. The CONSORT statement: revised
recommendations for improving the quality of reports of
parallel-group randomised trials. Lancet 2001;357:1191e4.
[13] Moher D, Altman DG, Schulz KF, Elbourne DR. Opportunities and
challenges for improving the quality of reporting clinical research:
CONSORT and beyond. CMAJ 2004;171:349e50.
[14] Plint AC, Moher D, Morrison A, Schulz K, Altman DG, Hill C, et al.
Does the CONSORT checklist improve the quality of reports of
randomised controlled trials? A systematic review. Med J Aust
2006;185:263e7.
[15] Egger M, Ju
¨
ni P, Bartlett C. Value of flow diagrams in reports of
randomized controlled trials. JAMA 2001;285:1996e9.
[16] Moher D, Cook DJ, Eastwood S, Olkin I, Rennie D, Stroup DF.
Improving the quality of reports of meta-analyses of randomised
controlled trials: the QUOROM statement. Quality of Reporting of
Meta-analyses. Lancet 1999;354:1896e900.
[17] Bossuyt PM, Reitsma JB, Bruns DE, Gatsonis CA, Glasziou PP,
Irwig LM, et al. Towards complete and accurate reporting of studies
of diagnostic accuracy: the STARD initiative. Ann Intern Med
2003;138:40e4.
[18] Vandenbroucke JP, von Elm E, Altman DG, Gøtzsche PC,
Mulrow CD, Pocock SJ, et al for the STROBE Initiative. Strengthen-
ing the Reporting of Observational Studies in Epidemiology
(STROBE): explanation and elaboration. PLoS Med 2007;4:e297.
DOI:10.1371/journal.pmed.0040297.
[19] Vandenbroucke JP, von Elm E, Altman DG, Gøtzsche PC,
Mulrow CD, Pocock SJ, et al for the STROBE Initiative. Strengthen-
ing the Reporting of Observational Studies in Epidemiology
(STROBE): explanation and elaboration. Ann Intern Med 2007;
147:W163e94.
[20] Vandenbroucke JP, von Elm E, Altman DG, Gøtzsche PC,
Mulrow CD, Pocock SJ, et al for the STROBE Initiative. Strengthen-
ing the Reporting of Observational Studies in Epidemiology
348 E. von Elm et al. / Journal of Clinical Epidemiology 61 (2008) 344e349