A schematic shows data collected from multiple patients with similar diagnoses but without any comparison or control group. What type of study design does this represent?

A case series is a type of observational, descriptive study that describes the characteristics of multiple patients with a similar disease or condition without a comparison group. Unlike a case report (single patient), a case series involves multiple patients and can identify patterns, clinical features, or outcomes. It lacks a control group and randomization, so it cannot establish causation or relative risk. Case series are useful for generating hypotheses, describing rare conditions, and identifying early signals of treatment effects or adverse events. They sit near the bottom of the evidence hierarchy but above anecdote or expert opinion.