OBJECTIVE: This paper presents a unified framework for assessment of the methodological quality of analytic study designs. STUDY DESIGN AND SETTING: A systematic review of 393 methodological quality assessment tools that updated a previous assessment with 100 tools. Tool items were extracted, examined and reworded. Bias domains and finally methodological standards to be fulfilled were defined. RESULTS: There were 36 unique methodological safeguards that were categorized into seven methodological standards to be fulfilled in the MASTER scale. These methodological standards reflect initial and ongoing equivalence in particular areas, including equal recruitment, equal retention, equal ascertainment, equal implementation, equal prognosis, sufficient analysis, and temporal precedence. CONCLUSION: This approach unifies existing methods for methodological quality assessment and will be useful for (1) clinical researchers when a bias assessment of clinical research studies is required across analytical designs, (2) promoting a unified framework for bias assessment.