From the Guidelines
A study with only 2 cases would be classified as a case series, which is a descriptive observational study design. This classification is based on the guidelines for reporting recommendations for tumor marker prognostic studies, as outlined in the Journal of the National Cancer Institute 1. A case series involves detailed descriptions of specific instances of a disease, condition, or treatment outcome without a comparison group. With just 2 cases, this would be considered a very small case series, which has significant limitations.
- Key characteristics of a case series include:
- Detailed descriptions of specific instances of a disease, condition, or treatment outcome
- Lack of a comparison group
- Limited ability to establish causation or effectiveness of interventions
- According to the study design guidelines, it is essential to:
- State the method of case selection, including whether prospective or retrospective and whether stratification or matching was used 1
- Precisely define all clinical endpoints examined
- List all candidate variables initially examined or considered for inclusion in models
- Give rationale for sample size; if the study was designed to detect a specified effect size, give the target power and effect size Case series are useful for generating hypotheses about rare conditions or unusual presentations but cannot establish causation or effectiveness of interventions. They sit low in the hierarchy of evidence, above only expert opinion and case reports (which describe single cases) 1. The small sample size of 2 cases means any findings would be preliminary at best and would require validation through larger, more rigorous study designs such as cohort studies, case-control studies, or randomized controlled trials before clinical recommendations could be made.
From the Research
Study Design for 2 Cases
If you have 2 cases, the study design is likely a case series.
- A case series is a descriptive study that follows a group of patients who have a similar diagnosis or who are undergoing the same procedure over a certain period of time 2.
- In a case series, patients are sampled based on a specific outcome, and the study may include patients with or without specific exposures 3.
- The results of a case series can generate hypotheses that are useful in designing further studies, but no causal inferences should be made regarding the efficacy of the investigated treatment 2.
Key Characteristics of a Case Series
- A case series may include a small number of patients, and the study design is often used to describe a new or rare condition 4.
- The study design is descriptive, and the results are often reported as proportions or rates of outcomes 4.
- Case series can be used to identify potential associations between exposures and outcomes, but these associations should be interpreted with caution due to the lack of control for confounding variables 3.
Distinguishing Case Series from Other Study Designs
- A case series can be distinguished from a cohort study, which samples patients based on exposure and follows them over time to assess the occurrence of outcomes 3.
- A case series can also be distinguished from a randomized controlled trial, which involves the random allocation of patients to treatment or control groups 2.