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The structured abstract should enable the reader to assess the study hypothesis and methods quickly and easily. The context for the study question and the hypothesis (objective) should be clearly stated (eg, “To determine whether enalapril reduces left ventricular mass …”), the study design and and setting from which the sample was drawn described, and the main outcome measures explained. The results should include some explanation of effect size, if appropriate, with and used to describe the results. The conclusions should follow from the results without overinterpreting the data. Abstract format is too brief to permit detailed explanation of statistical ...Less

A case series describes characteristics of a group of patients with a particular disease or patients who have undergone a particular procedure. A case series may also involve observation of larger units such as groups of hospitals or municipalities, as well as smaller units such as laboratory samples. Case series may be used to formulate a case definition of a disease or describe the experience of an individual or institution in treating a disease or performing a type of procedure. Case series should comprise consecutive patients or observations seen by the individual or institution to minimize selection bias. A case ...Less

Case-control studies, which are always retrospective, compare those who have had an outcome or event (cases) with those who have not (controls). Cases and controls are then evaluated for exposure to various risk factors and thus should never be selected on the basis of their exposure to the risk factors under investigation. Cases and controls generally are matched according to specific characteristics (eg, age, sex, or duration of disease) to reduce confounding by these variables. However, if the matched variables are inextricably linked with the exposure of interest (not necessarily with the disease or outcome of interest), matching may confound ...Less

A prospective cohort study follows a group or cohort of individuals who are initially free of the outcome of interest. Individuals in a cohort generally share some underlying characteristic, such as age, sex, or exposure to a risk factor. Some studies may comprise several different cohorts. The study is usually conducted for a predetermined period, long enough for some members of the cohort to develop the outcome of interest. Individuals who developed the outcome are compared with those who did not. The report of the study should include a description of the cohort and the length of follow-up, what independent ...Less

Although a treatment or screening technique may be shown to be effective in an RCT, recommending it in general practice would not necessarily be rational. Such interventions may be prohibitively expensive, or they may benefit only a small number of people at the expense of a large number of people, or they may lead to significant “downstream” costs that would eventually negate any immediate savings or benefit. Thus, it is possible that interventions that appear less effective may actually lead to the greatest societal benefits over the long term.Cost-effectiveness and cost-benefit analyses comprise a set of mathematical techniques to model ...Less

In a crossover trial, participants receive more than 1 of the treatments under investigation, usually in a randomly determined sequence, and with a prespecified amount of time (a “washout period”) between sequential treatments. The participants and the investigators are generally blinded to the treatment assignment (double-blinded). This experimental design is often used for evaluating drug treatments. Each participant serves as his or her own control, thereby eliminating variability when comparing treatment effects and reducing the sample size needed to detect a significant effect. Most considerations of parallel-design randomized trials apply. Rather than indicating which participants were assigned to which condition, ...Less

Whether the hypothesis was supported or refuted by the results should be addressed. The study result should be placed in context of published literature. The limitations of the study should be discussed, especially possible sources of bias and how these problems might affect conclusions and generalizability. Evidence to support or refute the problems introduced by the limitations should be provided. The implications for clinical practice, if any, and specific directions for future research may be offered. The conclusions should not go beyond the data and should be based on the study results and limited to the specific population represented by ...Less

It is sometimes desirable to compare a less expensive treatment or intervention against a treatment or intervention that is already known to be effective. In these cases, it would be unethical to expose participants to an inactive placebo. Thus, these trial designs assess whether the treatment or intervention under study (the “new intervention”) is no worse than an existing alternative (the “active control”).In equivalence and noninferiority trials, authors must prespecify a margin of noninferiority (Δ), within which the new intervention can be assumed to be no worse than the active control. There are a number of methods for arriving at ...Less

UPDATE: Although our style manual recommends (Section 20.9, page 888 in the print) that "[expressing] P to more than 3 significant digits does not add useful information to P<.001," in certain types of studies (particularly GWAS [genome-wide association studies] and other studies in which there are adjustments for multiple comparisons, such as Bonferroni correction, and the definition of level of significance is substantially less than P<.05) it may be important to express P values to more significant digits. For example, if the threshold of significance is P<.0004, then by definition the P value must be expressed to at least 4 ...Less

Each portion of the manuscript should contribute to the reader’s understanding of why and how the study was done and should help persuade the reader that (1) the hypothesis or study question is clearly stated, carefully considered, and important, (2) the methods are designed to answer the question and the analysis is appropriate, (3) the results are credible, and (4) the implications are placed in context and the limitations do not preclude interpretation of the results.Words used herein that are defined in the glossary (see , Glossary of Statistical Terms) are given in a different font.The structured abstract should enable ...Less