Study design
Case series
A descriptive report of several patients or events without a formal comparison group.
What it means
Definition
Case series. A case series can flag unusual patterns or potential harms, but it cannot estimate treatment effect reliably. It lacks the comparison needed to separate intervention effects from chance or selection.
What it can tell you
It can show how close the evidence is to a real human health outcome.
What it cannot tell you
It cannot prove that a treatment works in people unless human outcome data are available.
Headline trap
Why this matters in headlines
Several dramatic recoveries after a treatment do not prove the treatment caused recovery.