Newcastle-Ottawa Scale (NOS)

The Newcastle-Ottawa Scale (NOS) is a star-rating quality-assessment tool used to appraise non-randomized studies — specifically cohort studies and case-control studies — for inclusion in systematic reviews and meta-analyses. It was developed by G.A. Wells, B. Shea, D. O’Connell, J. Peterson, V. Welch, M. Losos, and P. Tugwell through a collaboration between the University of Newcastle (Australia) and the University of Ottawa (Canada), the source of its name, and is now maintained and published by the Ottawa Hospital Research Institute (OHRI).

Last verified August 25, 2026 against the Ottawa Hospital Research Institute’s NOS page and the Cochrane Handbook for Systematic Reviews of Interventions.

Why It Exists

Randomized controlled trials have well-established risk-of-bias tools, notably Cochrane’s RoB 2. Observational studies needed an equivalent that reviewers could apply consistently, and the NOS fills that gap using a star-rating system built around three domains common to cohort and case-control designs: how the study groups were selected, how comparable those groups were, and how exposure or outcome was ascertained. The scale exists in two parallel versions — one for cohort studies, one for case-control studies — sharing the same three-domain structure but with different items within Selection and the Outcome/Exposure domain, reflecting the different logic of each design.

Structure: 9 Stars Across 3 Domains

Domain	What it assesses	Max. stars
Selection	Representativeness/definition of the study groups, how they were ascertained, and (cohort studies) whether the outcome was absent at baseline	4
Comparability	Whether the study controlled for the review team’s pre-specified most important confounder, plus any additional confounder	2
Outcome (cohort) / Exposure (case-control)	How outcome or exposure was ascertained, adequacy of follow-up or non-response reporting	3

Each item earns at most one star, awarded only when a study meets the specific criterion on the relevant NOS coding form — there is no partial credit within an item. The scale does not publish an official good/fair/poor cutoff; the commonly used AHRQ-derived threshold convention (roughly: 3–4 Selection stars, 1–2 Comparability stars, and 2–3 Outcome/Exposure stars for “good” quality) is a widely applied convention rather than part of the official NOS documentation itself.

NOS vs. ROBINS-I vs. GRADE

These three tools are frequently confused but operate at different levels and are not interchangeable:

Tool	Appraises	Output
Newcastle-Ottawa Scale (NOS)	Individual cohort or case-control studies	Star count per domain (max 9)
ROBINS-I	Individual non-randomized studies of interventions	Domain-level risk-of-bias judgment (Low/Moderate/Serious/Critical/No information)
GRADE	The body of evidence for a specific outcome across all included studies	Certainty rating (High/Moderate/Low/Very low)

A review generally uses NOS or ROBINS-I — not both — to appraise the same set of individual studies; GRADE then operates one level up, synthesizing the already-appraised evidence base into an overall certainty rating for a given outcome. The Cochrane Handbook’s general preference for domain-level, non-composite risk-of-bias reporting is one reason Cochrane reviews of non-randomized studies of interventions now typically use ROBINS-I rather than the NOS; the NOS nonetheless remains extremely widely used outside formal Cochrane reviews, particularly in general biomedical and public-health meta-analyses of cohort and case-control evidence, for its speed and ease of training reviewers to apply it consistently.

Where NOS Fits in a Systematic Review

The NOS appraises individual included studies; it does not itself tell you what to report about that appraisal in the finished review — that is the job of a reporting guideline. A meta-analysis of observational studies commonly pairs NOS scoring with the MOOSE checklist, since MOOSE specifies what to report about quality assessment and confounding without being a quality-appraisal tool itself. See CASRAI’s full Newcastle-Ottawa Scale guide for domain-by-domain scoring guidance and a worked example.

Why This Matters for Research Administration

For staff supporting a systematic-review or evidence-synthesis team, confirming which quality-appraisal tool a protocol specifies — and whether that choice matches the target journal’s or funder’s expectations — is a methods-section detail worth checking before data extraction begins, since NOS, ROBINS-I, and design-specific tools like CASP are not interchangeable and a late switch can mean re-appraising every included study.

Frequently Asked Questions

What does NOS stand for?

The Newcastle-Ottawa Scale, named for the University of Newcastle (Australia) and the University of Ottawa (Canada), whose researchers developed it.

What study designs does the NOS appraise?

Cohort studies and case-control studies. There is no official OHRI-endorsed version for cross-sectional studies, though unofficial adaptations circulate in the published literature.

Is a higher NOS star total always “better”?

Not necessarily as a hard cutoff. The Cochrane Handbook cautions against collapsing risk-of-bias assessment into a single composite score, since it can mask which specific domain is driving a low rating; reporting domain-level stars alongside any threshold used is the more defensible approach.

Is the NOS the same as MOOSE?

No. MOOSE is a reporting guideline that specifies what a meta-analysis of observational studies should report, including about quality assessment; the NOS is the tool commonly used to actually perform that quality assessment on each included study.

References

• Ottawa Hospital Research Institute, Newcastle-Ottawa Scale coding manuals and forms, ohri.ca.
• Wells, G.A. et al., “The Newcastle-Ottawa Scale (NOS) for assessing the quality of nonrandomised studies in meta-analyses.”
• Cochrane Handbook for Systematic Reviews of Interventions, chapter on risk-of-bias assessment for non-randomized studies.