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 InstituteRosemary 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

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Meta-analysis ResultsGraphical Output

Prevalence Rate.01 .015 .02 .025

Combined

Casteldaccia 40+

Ponza 40+

Egna-Neumarkt 40+

Baltimore 40+

Melbourne 40+

Beaver Dam 43+

Blue Mountain 49+

Ireland 50+

Framingham 52+

Rotterdam 55+

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

Study Area

Macular

Degeneraton

Glaucoma

Cataract

Diabetic

Retinopathy

Refractive

Error

Framingham, Massachusetts X X X X X

Great Britain X X

Gisborne, New Zealand X X

Barbados X X X X X

Baltimore, Maryland X X X

Beaver Dam, Wisconsin X X X X X

Blue Mountains, Australia X X X X X

Visual Impairment Project, Australia X X X X X

Proyecto Ver, Baltimore X X

Salisbury, Maryland X X

Melbourne, Australia X

St. Lucia, West Indies X

West of Ireland X

Rotterdam, The Netherlands X X X

Casteldaccia, Sicili X X

Ponza, Italy X

Egna-Neumarkt, Italy X

Tanzania, Africa X

Southern Wisconsin X

San Luis Valley, Colorado X X

San Antonio, Texas X

New Jersey X

Western Samoa X

Mexico City and San Antonio, Texas X

Baltimore, Maryland (Screening Project) X

Los Angeles, California, (UCLA Mobile Eye Clinic) X

Methodological ApproachMeta-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