Type of studies
Type of Clinical literature
Primary literature: Primary sources are original materials. It is authored by researchers, contains original research data, and is usually published in a peer-reviewed journal.For Example, a clinical trial done by a pharmaceutical company would be a classic example of a primary literature. Another example. original research results in journals (e.g. NEJM).
It is important to note that the general classification of ’literature review’ has three varieties: Narrative review, qualitative systematic review, and quantitative systematic review (meta-analysis).
A systematic review is defined as “a review of the evidence on a clearly formulated question that uses systematic and explicit methods/criteria to identify, select and critically appraise relevant primary research, and to extract and analyze data from the studies that are included in the review.”
Meta-analysis is a quantitative, formal, and statistical analysis used to systematically assess previous research studies that combine the results of multiple scientific studies o derive conclusions about that body of research. Meta-analyses can be performed when there are multiple scientific studies addressing the same question, with each individual study reporting measurements that are expected to have some degree of error.
Randomized clinical trials
Interventional in nature.
Observational, study groups are divided by treatment, followed over time, and the outcomes compared. Exposure to the treatment group must precede the outcome of interest. The exposures are defined before looking at the existing outcome data to see whether exposure to a risk factor is associated with a statistically significant difference in the outcome development rate.
Cohort studies can be performed prospectively or retrospectively.
Retrospective cohort studies are NOT the same as case-control studies.
Retrospective in nature. Analytically, case-control studies work backward from outcome to exposure.
Groups are divided pre- and post implementation of some practice. Often used for evaluating the impact of a protocol.
A cross-sectional study is defined as a type of observational study (Descriptive) that analyzes data of variables collected at one given point in time across a sample population or a pre-defined subset. Both exposure and outcomes are assessed simultaneously (at one point in time).
Often referred to as a snapshot study or prevalence study (e.g. survey).
A case study, also known as a case report, is an in depth or intensive study of a single individual or specific group, while a case series is a grouping of similar case studies / case reports together. They are generally descriptive studies based on qualitative data e.g. observations, interviews, questionnaires, diaries, personal notes or clinical notes.
There are many classifications for case series: (El-Gilany AH. What is case series?. Asp Biomed Clin Case Rep. 2018 Aug 17;1(1):10-15.)
A. Informal vs. Formal Case Series:
Informal case series: Cases are selected for specific reasons: best case, worse case, significant variations. The format of this kind of case series is: introduction; case 1, case 2, case 3, etc. (each case is presented as a short case description); discussion (cases will be compared to one another, related cases to the current literature, implications of the findings, teaching points and what changes in clinical practice this might engender).
Formal case series: Include all cases of a specific type, or with specific selection criteria, presented more like a cohort study than a single case report and its format is introduction methods, results, and discussion/conclusions.
B. Consecutive vs. Non-Consecutive Case Series:
Consecutive case series: Includes all eligible patients identified by the researchers during the study period. The patients are treated in the order in which they are identified. Consecutiveness increases the quality of the case series.
Non-consecutive case series: Includes some, but not all, of the eligible patients identified by the researchers during the study period.
C. Exposure or Outcome-Based Sampling:
Exposure-based sampling: Include all patients treated and have specific outcomes or adverse events. Sampling is based on both a specific outcome and the presence of specific exposure.
Outcome-based sampling: Includes patients with the specific outcome regardless of exposure. Thus neither absolute risk nor relative risk can be calculated. Selection is based only on a specific outcome, and data are collected on previous exposures.
Questions/elements to be included in the presentation of the cases (Abdel-Hady El-Gilany.2018):