Strata Academy

Newcastle–Ottawa Scale (NOS) explained – cohort and case–control studies

Selection, comparability, and exposure/outcome domains; star ratings; limits for intervention causal inference

1. What is the Newcastle–Ottawa Scale?

NOS is a widely used quality assessment tool for non-randomised cohort and case–control studies, particularly in systematic reviews and meta-analyses of observational data.

It assigns up to nine stars across three domains: selection of study groups, comparability of groups, and ascertainment of exposure (case–control) or outcome (cohort).

NOS was not designed as a risk-of-bias tool for causal intervention inference – it assesses selected observational quality features that correlate imperfectly with validity.

When appraising a meta-analysis, note whether authors used NOS, ROBINS-I, or an ad hoc checklist – tool choice affects how much to trust pooled estimates.

2. Three NOS domains

Work through each domain independently – do not collapse to a single gut-feel star count without domain-level notes.

Selection stars reward representativeness of exposed and non-exposed cohorts, adequate case definitions, and community-based controls in case–control designs.

Comparability stars require control for key confounders in design or analysis. One star for controlling age/sex; a second often requires additional important prognostic factors.

Outcome or exposure stars reward blinded assessment, independent record linkage, or adequate follow-up duration in cohort studies.

• Selection (max 4 stars) – representativeness, non-exposed selection, exposure ascertainment
• Comparability (max 2 stars) – adjustment for key confounders
• Outcome/exposure (max 3 stars) – blinded assessment; adequate follow-up (cohort)

3. Cohort vs case–control variants

NOS has design-specific prompts. Case–control studies emphasise adequate case definition, community controls rather than hospital controls, and reliable exposure ascertainment.

Cohort studies emphasise that the outcome was not present at start, follow-up duration was long enough for the disease biology, and loss to follow-up was acceptable.

Read the version of NOS your review team or journal specifies – wording differs slightly between adaptations used in different meta-analysis traditions.

Case–control designs are efficient for rare outcomes but prone to recall and selection bias – star ratings should reflect those threats.

4. NOS vs ROBINS-I

For causal questions about interventions in non-randomised data, Cochrane increasingly favours ROBINS-I because it targets intervention bias domains explicitly with signalling questions.

NOS remains common in historical meta-analyses, environmental epidemiology, and some specialty journals. When appraising a review, note which tool was used and whether it matched the causal question.

High NOS stars do not prove causation – only that common observational quality items were partially addressed.

In coursework, if the question is 'does treatment X cause outcome Y?', prefer ROBINS-I; if describing prognostic cohort quality in a non-intervention context, NOS may be acceptable.

5. Interpreting star counts

Reviews sometimes dichotomise ≥7 stars as 'high quality' – thresholds are arbitrary and field-specific.

Report stars per domain and justify cut-points if you use them in meta-analysis sensitivity analyses.

A study can score well on selection but poorly on comparability if key confounders were unmeasured – domain detail matters more than total stars.

Do not exclude studies from narrative synthesis solely on NOS thresholds without examining whether exclusion changes conclusions directionally.

6. Common mistakes

Applying NOS to RCTs or diagnostic accuracy studies.

Giving comparability stars without checking which covariates were adjusted and whether they are sufficient.

Ignoring loss to follow-up in cohort studies – especially informative censoring related to outcome.

Treating NOS as interchangeable with ROBINS-I in intervention meta-analyses without supervisor approval.

7. StrataResearch and NOS

Cohort and case–control manuscripts may receive NOS-aligned secondary appraisal alongside design-appropriate primary frameworks.

Intervention comparisons route primarily to ROBINS-I; NOS context may appear when design is prognostic rather than interventional.

Upload observational papers via quick analysis to see which framework StrataResearch selects before completing manual worksheets.

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