Introduction to Newcastle Ottawa Scale
The
Newcastle Ottawa Scale (NOS) is a widely used tool for assessing the quality of non-randomized studies, particularly
cohort and
case-control studies. Developed by researchers at the Universities of Newcastle, Australia and Ottawa, Canada, the scale evaluates studies based on three broad perspectives: selection, comparability, and outcome.
Selection Criteria
The selection criteria focus on how participants are chosen and include four questions: Representativeness of the exposed cohort: Is the cohort truly representative of the average population?
Selection of the non-exposed cohort: Is the non-exposed cohort drawn from the same population as the exposed cohort?
Ascertainment of exposure: How was exposure ascertained? Was it through secure records or structured interviews?
Demonstration that outcome of interest was not present at start of study: Was it made clear that the outcome was not present in participants at the beginning?
Comparability Criteria
The comparability criteria assess the study’s ability to control for confounding factors. This section includes one primary question: Comparability of cohorts on the basis of the design or analysis: Were the cohorts comparable based on the design and analysis? Was there control for confounders?
Outcome Criteria
The outcome criteria evaluate the methods used to measure the results and include three questions: Assessment of outcome: How was the outcome assessed? Was it through independent blind assessment or record linkage?
Was follow-up long enough for outcomes to occur? This question checks whether the follow-up period was sufficient to observe the outcomes of interest.
Adequacy of follow-up of cohorts: Was the follow-up complete? If not, were the reasons for loss to follow-up described and were those lost to follow-up accounted for in the analysis?
Scoring System
Each question in the NOS is scored, with higher scores indicating better quality. The maximum score for a cohort study is 9, while for a case-control study, it is 8. The scores help researchers and clinicians determine the reliability and validity of the study findings.Application in Epidemiology
In
epidemiology, the NOS is particularly useful for systematic reviews and meta-analyses. It helps in evaluating the quality of primary studies and ensures that the evidence synthesized is robust and reliable. The scale's comprehensive criteria make it a preferred choice for assessing observational studies.
Strengths and Limitations
One of the main strengths of the NOS is its simplicity and ease of use. It provides a structured approach to assess study quality. However, it also has limitations. For instance, the scale may not capture all aspects of study quality, such as the appropriateness of the statistical analysis. Additionally, the subjective nature of some criteria may lead to variability in scoring among different reviewers.Conclusion
The Newcastle Ottawa Scale is a valuable tool in the field of epidemiology for assessing the quality of non-randomized studies. By focusing on selection, comparability, and outcome criteria, the NOS provides a standardized method to evaluate the robustness of study findings, aiding researchers in conducting high-quality systematic reviews and meta-analyses.
