Research Synthesis of Population-Based Prevalence Studies ORC Macro Benita J. O’Colmain, M.P.H. Wanda Parham, M.P.A. Arlen Rosenthal, M.A. Adrienne Y. Semidey, M.A. National Eye Institute Rosemary Janiszewski, M.S.

Research Synthesis Synthesis of research findings offers the potential to: Identify areas of agreement Increase statistical power Identify areas needing more research Fosters a greater sensitivity to the currently available scientific evidence

Purposes of Research Synthesis State-of-the-art literature review Presents and analyzes available data, framing results so they can be translated into practice and policy Logical framework for systematic and objective review of data (scientific results) Establish what is known prior to further research

Research Synthesis Essential Steps Specification of objectives, hypotheses, scope, and methods of systematic review Compilation of a comprehensive set of relevant primary studies Assessment of the methodological quality of the studies Identification of a common set of definitions of outcome, explanatory and confounding variables Extraction of a standardized set of outcome measures and of study subjects and characteristics

Meta-analysis Subset of research synthesis Statistical analysis of a large collection of analysis results from individual studies for the purpose of integrating the findings (Glass, 1976) May produce more generalizable results Helps to explore variation between studies and assess factors which might modify effects

Purposes of Meta-analysis Statistical methods for combining results Fixed vs. random effects models Gain in statistical power for estimates Reduces the effect of random error More precise summary estimate Resolve issues related to conflicting results from studies Formal/informal models for heterogeneity

Meta-analysis Essential Steps Use appropriate and clearly stated models Explore and allow for all important sources of variation Report confidence intervals for pooled estimates Evaluate tests of heterogeneity Clear presentation of results to enable critical appraisal and replication

Guiding Principles In a research synthesis/meta-analysis, individual study results are the raw data Systematic reviews are retrospective observational studies and thus are potentially subject to the same biases as other retrospective observational studies Need an a priori protocol for data selection and analysis

Population-Based Epidemiological Eye Studies Conducted over the past 20 years Framingham, 1980 Assess prevalence of eye disease based on results of clinical examinations Evaluate vision impairment and blindness Study risk factors associated with eye disease

Objectives of Research Synthesis of Population-Based Eye Studies Estimate prevalence of blindness and visual impairment (all cause) Estimate prevalence of: Age-related macular degeneration Diabetic retinopathy Glaucoma Cataracts Refractive error (myopia and hyperopia) Estimate attributable blindness and visual impairment due to these diseases

Methodological Approach Research Synthesis Literature review Identify appropriate studies for each disease Determine inclusion/exclusion criteria Review case definitions for each disease Develop consistent criteria across studies Use uniform age categories for prevalence estimates (5 year CDC standard)

Preliminary List of Studies

Methodological Approach Meta-analysis Obtain raw prevalence data from each study according to standardized case definitions and uniform age groups Determine appropriate models Review graphical and statistical output Collapse cells where data is sparse Assess heterogeneity

Current Status Completed meta-analysis for age- related macular degeneration Derived age-specific prevalence rates: Geographic and neovascular AMD Drusen >125 Blindness due to AMD Meta-analysis for diabetic retinopathy in progress

Next Steps Complete meta-analysis for other diseases Determine current burden of: Each disease National and State level by race and gender Blindness and vision loss due to each disease Future projections Publish results