STROBE Statement

STrengthening the Reporting of OBservational Studies in Epidemiology

Iveta Simera

The EQUATOR NetworkCentre for Statistics in Medicine, Oxford, UK

April 2012

Observational studies

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Many types of studies - right research design depends on the question we ask

Observational studies•Large proportion of research•Can be valuable (e.g. AE) but also many disadvantages (confounding, bias)

Without comparison group – descriptive:

(do not try to qualify the relationships but give us a perspective of what is happening in the population, prevalence or experience of a group)

•Case reports, case series, qualitative studies, some cross-sectional studies (surveys)

With comparison group - analytical

(attempts to qualify relationship between two factors – effect of an intervention / exposure on an outcome)

•Cohort studies, case-control studies, some cross-sectional studies

Difference in analytical designs

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Experimental studies:

•Researcher manipulates exposure by allocating participants to Intervention (Exposure) group

Observational studies with a comparison group:

•Researcher simply measures the exposure or treatments of the groups

•These studies all include matched groups of participants

•They assess associations between exposures and outcomes

Cohort studies

• Cohort = group of Roman soldiers• Start with exposure (variable) then follow for

outcome

• Data are obtained from groups who have been exposed or not exposed to the factor of interest

• Best for study the effect of predictive risk factors on an outcome

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Case-control studies

• Patients with a certain outcome or disease and an appropriate group of controls without the outcome or disease are selected (usually with careful consideration of choice of controls, matching)

• Information is obtained on whether the participants have been exposed to the factor under investigation

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Cross-sectional studies

• Examine the relationship between diseases (or other health-related characteristics) and other variable of interest as they exist in a defined population at one particular time (outcomes and exposures are both measured at the same time)

• Best for quantifying the prevalence of a disease or risk factor, and for quantifying the accuracy of a diagnostic test

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7Grimes, DA, Schulz KF. An overview of clinical research:the lay of the land. Lancet 2002; 359; 57-61.

STROBE

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STROBE Statement

• Guidance on how to report observational studies well (which is rare!)– Focus on 3 main study designs: cohort, case-control, cross-

sectional studies• Published in Oct 2007: short paper and E&E • Adopted by many journals

Find it on:www.equator-network.orgwww.strobe-statement.org

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Three STROBE extensions (1)

• STREGA (2009)– reporting of genetic association studies

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Three STROBE extensions (2)

• STROBE – ME (Oct 2011)– Reporting molecular epidemiology (biomarker studies)

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Three STROBE extensions (3)

• STROBE abstract- Reporting observational

studies in conference abstracts (online draft)

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STROBE

• Checklist with 22 items– Heading (where in paper), item No– Recommendation, divided into:

cohort, case-control, cross-sectional study - where different

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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

Objectives3. State specific objectives, including any prespecified

hypothesis

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Methods:Study Design

4. Present key elements of study design early in the paper (what design, what was compared, which controls and why...etc)

Setting

5. Describe the setting, locations, and relevant dates, including periods of recruitment, exposure, follow-up, and data collection

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Methods - continuedParticipants6.a) Cohort study:

• eligibility criteria• sources and methods of participant selection • follow-up methods

Case-control study: • eligibility criteria• sources and methods of case ascertainment and control selection• rationale for the choices of cases and controls

Cross-sectional study: • eligibility criteria• sources and methods of participant selection

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Methods - continuedParticipants

6.b) Cohort study:For matched studies, give matching criteria and number of exposed and unexposed

Case-control: For matched studies, give matching criteria and the number of controls per case

Variables7. Clearly define all outcomes, exposures, predictors,

potential confounders, and effect modifiers. Give diagnostic criteria, if applicable

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Methods - continuedData 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

* Give information separately for cases and controls in case-control studies and, if applicable, for exposed

and unexposed in cohort and cross-sectional studies

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Methods - continuedBias

9. Describe any efforts to address potential sources of bias

(ie systematic deviation of a result from the true value) e.g.: recall bias, detection bias, interviewer bias, selection bias

Very important in observational studies!

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Methods - continuedStudy size

10. Explain how the study size was arrived at

(should be large enough to arrive at a point estimate with a reasonably narrow confidence interval)

Quantitative variables11. Explain how quantitative variables were handled in

the analyses. If applicable, describe which groupings were chosen and why

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Methods - continuedStatistical methods12.a) Describe all statistical methods, including those

used to control confounding

(≠bias, confounding: association true but caused by something else)

b) Describe any methods used to examine subgroups and interactions

c) Explain how missing data were addressed

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Methods - continuedStatistical methods - continued12.d) Cohort study:

If applicable, explain how loss to follow-up was addressed

Case-control: If applicable, explain how matching of cases and controls was

addressed

Cross-sectional: If applicable, describe analytical methods including sampling

strategy

e) Describe any sensitivity analyses

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ResultsParticipants13. a) Report numbers of individuals at each stage of study - e.g., numbers potentially eligible, examined for eligibility, confirmed eligible, included in the study, completing follow-up, and analysed

b) Give reasons for non-participation at each stage

c) Consider use of a flow diagram

* Give information separately for cases and controls in case-control studies and, if applicable, for exposed and unexposed in cohort and cross-sectional studies

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Results - continuedDescriptive data14. a) Give characteristics of study participants (e.g. demographic, clinical, social) and information on exposures and potential confounders

b) Indicate number of participants with missing data for each variable of interest

c) Cohort study: Summarise follow up time (e.g. average and total amount)

* Give information separately for cases and controls in case-control studies and, if applicable, for exposed and unexposed in cohort and cross-sectional studies

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Results - continuedOutcome data15. Cohort study:

Report numbers of outcome events or summary measures over time

Case-control: Report numbers in each exposure category, or summary measures of

exposure

Cross-sectional: Report number of outcome events or summary measures

* Give information separately for cases and controls in case-control studies and, if applicable, for exposed and unexposed in cohort and cross-sectional studies

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Results - continuedMain results

16 a) Give unadjusted estimates and, if applicable, confounder-adjusted estimates and their precision (e.g. 95%CI). Make clear which confounders were adjusted for and why they were included

b) Report category boundaries when continuous variables were categorised

c) If relevant, consider translating estimates of relative risk into absolute risk for a meaningful time period

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Results - continuedOther analyses17. Report other analyses done, e.g. analyses of subgroups

and interactions, and sensitivity analyses

DiscussionKey results18. Summarize key results with reference to study objectives

Limitations19. Discuss limitations of the study, taking into account sources of potential bias or imprecision. Discuss both direction and magnitude of any potential bias

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Discussion - continuedInterpretation20. Give a cautious overall interpretation of

results considering objectives, limitations, multiplicity of analyses, results from similar studies, and other relevant evidence

Generalisability21. Discuss the generalisability (external validity) of the study results

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Other informationFunding22. 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

More detailed explanation:

Vandenbroucke JP, von Elm E, Altman DA, et al. Strengthening the Reporting of Observational Studies in Epidemiology (STROBE): Explanation and Elaboration. PLoS Med 4(10): e297.doi:10.1371/journal.pmed.0040297

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www.equator-network.org

www.strobe-statement.org