Meta Analysis MAE Course 2005

Meta-analysis The statistical combination and analysis of data from separate and independent studies to determine if there is an association between exposure and outcome and the size of this effect

Systematic Overview Vs. Overview Qualitative vs. quantitative Literature reviews Statistical power and precision

Focus of Meta-analysis Direction of the effect across studies Magnitude of the effect across studies Traditional methods (significance testing) not well suited

Steps in Conducting a Meta- analysis Background of the problem Develop a hypothesis Study Inclusion criteria Exclusion criteria Results –Participant characteristics –Interventions or exposures –Outcomes –Study design issues

Advantages of Meta-analysis Systematic means of reviewing findings of multiple studies May be used for a large number of studies May identify significant findings and relationships not apparent utilizing other means (i.e. “false negative” results)

Weaknesses of Meta-analysis “Comparability” of studies is sometimes difficult to assess Publication bias –“negative” studies may not be found Qualitative distinctions between studies Requires significant effort Methodological problems in original studies

Meta-analysis of studies on individual consumption of chlorinated drinking water and bladder cancer C M Villanueva, F Fernández, N Malats, J O Grimalt, M Kogevinas J Epidemiol Community Health 2003;57:166–173 Example

Study Objectives To evaluate whether consumption of chlorinated drinking water is associated with bladder cancer

Methods Bibliographic search 8 case control and cohort studies selected Looked at individual consumption of chlorinated water and incidence of bladder cancer Risk estimates extracted Meta analysis performed for two exposure levels and of the dose-response regression slopes

RESULTS

Ever Consumed Chlorinated Water

Long Term Chlorinated Water

Results This meta-analysis indicates that consumption of chlorinated drinking water is associated with a moderately increased risk of bladder cancer.

Some Limitations of This Meta- Analysis Publication bias Chemical mixtures Exposure categories among studies may be different