Inclusion & exclusion criteria (with PICO)

By Dr. Rafiq Muhammad, MD, PhD · Updated June 2026

Eligibility criteria are the gate every study passes through in a systematic review. Inclusion criteria say what a study must have to qualify; exclusion criteria say what disqualifies it. Get them specific enough that two reviewers screen the same way — and fix them in the protocol before you look at the evidence. The cleanest way to build them is from PICO.

Build criteria from PICO

PICO turns a fuzzy topic into a focused question and a set of eligibility rules. Each element becomes one or more criteria:

Variants exist for other question types — PEO and SPIDER for qualitative or mixed reviews — but the logic is the same: each element of the question maps to a criterion.

Inclusion vs exclusion — don’t just invert

Inclusion criteria define eligibility positively. Exclusion criteria remove studies that meet the inclusion criteria but carry a disqualifying feature — e.g. “trials where >20% of participants also had type 1 diabetes.” A common error is writing exclusions that merely restate the inverse of an inclusion (“include adults / exclude children”) — that’s redundant. Reserve exclusions for the genuine edge cases an inclusion rule can’t cleanly capture.

The make-or-break test: would two reviewers agree?

Eligibility criteria only work if they’re reproducible. Each one should be specific enough that two independent reviewers, screening the same record, reach the same verdict. Vague criteria (“recent studies,” “relevant populations”) collapse under real abstracts. Pilot your criteria on 20–30 records first, compare decisions, and tighten any rule that produced disagreement — before the main screen. And never loosen a criterion after seeing a striking study; that’s how selection bias creeps in.