To become expert in assessing the literature, you first have to know what’s out there. Simply browsing through a professional journal’s table of contents will reveal many different types and/or categories of articles.
In general, there are three categories of journal articles: (1) primary or original research studies, (2) secondary (or integrative) studies, and (3) 'other' articles. Primary studies report research first-hand. Secondary studies summarize and draw conclusions from primary studies. The 'other' category includes articles that are not research based, such as editorials and reports. Below is a description of the most common types of articles seen in the professional literature.
Primary studies report research first-hand. Primary or original research studies commonly found in professional journals includes laboratory experiments, clinical trials, case-control studies, cohort studies, survey research, methodological studies and evaluation research. Some journals also report conference proceeding and/or abstracts. For completeness, primary sources of research-based knowledge found outside the journal literature include graduate theses/dissertations and governmental or private research reports.
Experiments test interventions on animals or human volunteers in an artificial and controlled laboratory setting. Most laboratory animal research falls into this category, as do some elements of human drug trials (see below). Due to the nature of the subjects and setting, the findings derived from laboratory experiments cannot be widely generalized to actual patient or client populations.
A clinical trial involves one or more test treatments, at least one control treatment, specified outcome measures for evaluating the studied intervention, and a bias-free method for assigning patients to the test treatment. The treatment may be drugs, devices, or procedures studied for diagnostic, therapeutic, or prophylactic effectiveness. Control measures include placebos, active medicines, no treatment, dosage forms and regimens, historical comparisons, etc.
When randomization is used to assign patients to test or control treatments, the trials are referred to as randomized controlled trials. A randomized controlled trial is the most powerful type of clinical experiment and is the only design that can truly establish cause and effect relationships.
When randomization is not possible, researchers commonly use one of two methods to minimize the effect of extraneous variables: matched-pair analysis or the cross-over method. In a matched-pair analysis, subjects in both the experimental and control groups are ‘paired’ on the factor or factors of interest, e.g., age-matched controls. In the cross-over method, the subjects, upon completion of the course of one treatment, are switched to another (or none). In this manner, the subjects serve as their ‘own controls.’ Alternatively, statistical methods can be applied to balance out differences between groups on key extraneous variables. The most common statistical method used to control for difference between groups is analysis of covariance.
In order to minimize researcher bias, clinical trials are often ‘blinded.’ In the single-blind method, either the observer(s) or the subject(s) is kept ignorant of the group to which the subjects are assigned. In the double-blind method both the subjects and investigators are kept unaware of who is actually getting which specific treatment.
Controlled clinical trials which are planned and carried out by several cooperating institutions to assess certain variables and outcomes in specific patient populations are called multicenter clinical trials.
The FDA model for approval of new drugs, devices or procedures defines four phases of clinical trials, each with a different purpose:
Case-control studies start with the identification of persons with a disease of interest and a control (comparison, referent) group without the disease. The relationship of an attribute to the disease is examined by comparing diseased and non-diseased persons with regard to the frequency or levels of the attribute in each group.
A 'cohort' is a group of people clearly identified by one or more specific attributes. Cohort studies identify subsets of a defined population and follow them (either prospectively or retrospectively via records) in an attempt to determine distinguishing subgroup characteristics. These groups may or may not be exposed to factors hypothesized to influence the occurrence of a particular disease or other outcome. If the emphasis is on outcomes of exposures, procedures, or the occurrence of disease, this may be called a follow-up study.
Survey research employs either interviews or questionnaires to determines the presence, absence or degree to which selected attributes exist in a study population or in a representative sample at one particular time. The researchers examine and report findings, without deliberately intervening in the course of events. If the goal of the survey is to determine the prevalence of a disease or condition in a population, the term prevalence study is used.. The study is considered longitudinal if the attributes are assessed over a period of time.
Methodological studies involve the development and validation of measurement tools, methods or procedures used in practice and/or clinical research. Methodological studies may be designed to evaluate the accuracy of instrumentation or to assess the reliability and validity of psychological inventories (such as tests of motor development or intelligence tests) or surveys. The goal of a methodological study is not to evaluate the effects of treatment, but to develop valid and reliable instruments that can be used to answer clinical questions. If the study involves a small-scale test of methods or procedures to see if they will work on a larger scale, the term pilot study is used.
Evaluation research studies assess the efficacy or efficiency of processes or programs. They may include the evaluation of cost-effectiveness, the extent to which program objectives are met, or impact (outcomes assessment). If the intent is to determine the advantages or disadvantages, practicability, or capability of accomplishing a projected plan or project, the term feasibility study is used.
Conference proceeding and/or abstracts provide short summaries of in-progress or completed primary studies that are presented at conferences, but are not yet fully peer-reviewed for publication as complete articles. Conference proceeding and/or abstracts thus provide an 'early picture' of current research that is likely to appear later in one or more of the primary article forms listed above. Because selection processes are highly variable, the quality of conference proceeding and/or abstracts vary widely.
Secondary studies summarize and draw conclusions from primary studies. Secondary or integrative studies found in professional journals include review articles, meta-analyses, guidelines, decision analyses, and economic analyses. Outside the journal literature, books and book chapters are the most common secondary source materials.
There are two types of review articles: non-systematic (or journalistic) reviews and systematic reviews. Non-systematic or journalistic reviews provide a summary of evidence derived from primary studies that have been selected and synthesized according to the author's personal and professional perspective. Non-systematic reviews can cover a wide range of subject matter at various levels of completeness and comprehensiveness. Systematic reviews, on the other hand, provide summaries of related primary studies that have been searched for, evaluated, and selected and reported according to a rigorous and predefined methodology.
A meta-analysis is an integrative study that statistically combines the numeric results of several separate studies addressing the same question into a single estimate of their combined effect (commonly referred to as 'pooling data'). Typically, the results are presented as a graph depicting the individual and pooled odds ratios (with confidence intervals) for all studies included in the analysis.
A guideline (practice guideline; clinical practice guideline) is a systematically developed statement designed to help practitioners make appropriate decisions about health care in specific clinical circumstances. Good guidelines draw their conclusions from careful review of primary studies; some guidelines also involve quantitative assessment of alternative risks and benefits (see decision analysis).
Decision analyses use the results of primary studies to develop quantitative measures or estimates of the risks and benefits of alternative diagnostic or therapeutic options. These estimates are then used to develop a probability tree that can help health professionals and/or patients make informed choices about clinical management. Decision analysis also can be used to help develop clinical guidelines or consensus statements.
Economic analyses use the results of primary studies to say whether a particular course of action is a good use of resources. There are three common types of economic analyses appearing in the journal literature. Cost-effectiveness analysis assesses the ratio of costs to some naturally occuring health-related outcome (e.g. cost per additional year of survival). Cost-benefit analysis assesses costs relative to an outcome defined in actual monetary units (e.g., cost per dollar value of gained worker productivity). Cost-utility analysis assesses the ratio of costs to outcomes defined in units of 'utility' (e.g., cost per additional quality adjusted year of life).
A consensus statement is a statement developed by professionals via a group consensus process that is intended to advance health professional and/or public understanding of a targeted health problem, practice or issue. Examples include statements guiding professional practice; statements intended to help develop public policy; statements on insurance coverage of innovative therapies; or statements setting norms for evaluating clinical performance. Ideally, consensus statements are based on the evaluation of existing scientific evidence.
A case study is a report of a single clincal case (generally deemed 'interesting' or 'unusual' ). A case series is a description of a number of such cases. These studies usually are generated by the investigator's actual clinical encounters with patients with a given disease or disorder, which can be recent and/or historical in nature (based on records review). Ideally, a good case series (also called a review of cases) also cites the literature in which all other known cases were reported.
The term 'clinical protocol' has two meanings. In terms of published articles, a clinical protocol represents a standardized plan of treatment that typically includes various patient management contingencies and often is depicted or summarized using a decision algorithm. In clinical research, the term protocol refers to the detailed plan of study or study methods. For example, multicenter clinical trials employ rigorous written selection and treatment protocols to help assure study consistency.
An editorial is a short paper written by either the journal's editor(s) or by a guest editor that addresses an issue of interest to a given journal's readership. Editorials may serve to introduce topics covered within a journal issue, present oposing opinions, clarify positions, or provide readers with updated information on new methods or procedures.
A letter to the editor represents a (usually) brief communication submitted in response to an article previously published in the source journal. Some letters in some journal (e.g., Lancet) include extensive commentary with careful referencing to the literature and thus can serve as a valuable source to the primary research literature.
© 2001 Craig L. Scanlan