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Study Design and Statistics

Margaret A. Winker

and Stephen J. Lurie

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Subscriber: null; date: 20 June 2018


UPDATE: We will discontinue using quotation marks to identify parts of an article, but retain the capitalization; eg, This is discussed in the Methods section (not the “Methods” section). This change was made February 14, 2013.

The Methods section should include enough information to enable a knowledgeable reader to replicate the study and, given the original data, verify the reported results. Components should include as many of the following as are applicable to the study design:

  • Study design (see sections 20.2-20.7).

  • Year(s) (and exact dates if appropriate) when the study was conducted.

  • Disease or condition to be studied—how was it defined?

  • Setting in which participants were studied (eg, community based, referral population, primary care clinic), as well as geographic location and, if applicable, name of institution.

  • Individuals or other data studied—who or what was eligible; inclusion and exclusion criteria; if all participants were not included in each analysis, reason for exclusions; informed consent and approval by institutional review board or ethics committee when appropriate (see 5.8.1, Ethical and Legal Considerations, Protecting Research Participants' and Patients' Rights in Scientific Publication, Ethical Review of Studies and Informed Consent). If results for any of the participants have been previously described, provide citations for all reports or ensure that different reports of the same study can be easily identified (eg, by using a unique study name).

  • Any remuneration or other compensation for participants.

  • Intervention(s), including their length. In general, authors should provide sufficient detail to allow readers to replicate the interventions. This would also permit comparison with other studies. Treatment of any control or comparison groups should also be described in detail.

  • Outcomes and how they were measured, including reliability of measures and whether investigators determining outcomes were blinded to which group received the intervention or underwent the exposure.

  • Other variables and how they were measured—for example, demographic variables and risk factors for the disease. Such variables are often used to assess or adjust for confounding of the relationship between the dependent and independent variables.

  • Preliminary analyses: if the study is a preliminary analysis of an ongoing study, the reason for publishing data before the end of the study should be clearly stated, along with information regarding whether and when the study is to be completed. Authors should indicate whether such analyses were preplanned at the time the study began.

  • Source to obtain original or additional data if other than from the authors. For example, data tapes are often obtained from the US government; the source should be stated. The Web can be used to store or display data or information that could not be included in the manuscript. For information essential to the study, the information should be included in the manuscript, if at all possible. If this is not possible, the editors should request and consider retaining a copy, as websites and uniform resource locators (URLs) may change and become inaccessible. The source also may be listed in the acknowledgment.

  • Statistical methods, including procedures used for each analysis, what α level was considered acceptable, power of the study (which should have been calculated before the study was conducted to determine sample size), assumptions made, any data transformations or multiple comparisons procedures performed, steps used for developing a model in multivariate analysis, and pertinent references for statistical tests and type of software used. Authors should provide evidence that the data meet the assumptions of the statistical tests used. Test statistics should include degrees of freedom whenever applicable. It is always preferable for results to be presented in terms of point estimates and confidence intervals, which convey more information than do P values.

  • If the study has been registered in a central trial registry, the name of the registry and the trial number should be provided. (See 20.2, Randomized Controlled Trials.)

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