Generalised Evidence Synthesis Keith Abrams, Cosetta Minelli, Nicola Cooper & Alex Sutton Medical Statistics Group Department of Health Sciences, University of Leicester, UK CHEBS Seminar ‘Focusing on the Key Challenges’ Nov 7, 2003

Outline Why Generalised Evidence Synthesis? Bias in observational evidence Example: Hormone Replacement Therapy (HRT) & Breast Cancer Discussion

Why Generalised Evidence Synthesis? RCT evidence ‘gold standard’ for assessing efficacy (internal validity) Generalisability of RCT evidence may be difficult (external validity), e.g. CHD & women Paucity of RCT evidence, e.g. adverse events Difficult to conduct RCTs in some situations, e.g. policy changes RCTs have yet to be conducted, but health policy decisions have to be made Consider totality of evidence-base – (G)ES beyond MA of RCTs

Assessment of Bias in Observational Studies - 1 Empirical evidence relating to potential extent of bias in observational evidence (Deeks et al. 2003) Primary studies: –Sacks et al. (1982) & Benson et al. (2000) Primary & Secondary studies (meta-analyses): –Britton et al. (1998) & MacLehose et al. (2000) Secondary studies (meta-analyses): –Kunz et al. (1998,2000), Concato et al. (2000) & Ioannidis et al. (2001)

Assessment of Bias in Observational Studies - 2 Using a random effects meta-epidemiology model (Sterne et al. 2002) –Sacks et al. (1982) & Schultz et al. (1995) ~ 30% –Ioannidis et al. (2001) ~ 50% –MacLehose et al. (2000) ~ 100% Deeks et al. (2003) simulation study: comparison of RCTs and historical/concurrent observational studies –Empirical assessment of bias – results similar to previous meta-epidemiological studies –Methods of case-mix adjustment, regression & propensity scores fail to properly account for bias

Approaches to Evidence Synthesis Treat sources separately, possibly ignoring/downweighting some implicitly Bayesian approach & treat observational evidence as prior for RCTs & explicit consideration of bias: –Power Transform Prior –Bias Allowance Model Generalised Evidence Synthesis

Example – HRT HRT used for relief of menopausal symptoms Prevention of fractures, especially in women with osteoporosis & low bone mineral density BUT concerns have been raised over possible increased risk of Breast Cancer

HRT & Breast Cancer – RCT Evidence before July 2002 Source: Torgerson et al. (2002) OR % CI 0.67 to 1.39

HRT & Breast Cancer – Observational Evidence * Source: Lancet (1997) * Adjusted for possible confounders All ObservationalOR % CI 1.10 to 1.26 RCTsOR % CI 0.67 to 1.39

Use of Observational Evidence in Prior Distribution Case-ControlQuasi RCTs RCTs Cohort Prior Synthesis Empirical Evidence Bias

Following Ibrahim & Chen (2000) 0    1 is degree of downweighting  = 0  total discounting  = 1  accept at ‘face value’ Evaluate for a range of values of  Power Transform Prior

Power Transform Prior – Results 1

 * is unbiased true effect in observational studies  is bias associated with observational evidence  2 represents a priori beliefs regarding the possible extent of the bias Bias Allowance Model Following Spiegelhalter et al. 2003

Bias Allowance Model - Results Belief/SourceBias 22 OR95% CrIP(OR>1) ‘Face Value’ 0% to Total Discounting %%  to Sacks & Schultz 30% to Ioannidis 50% to MacLehose 100% to

HRT & Breast Cancer: Evidence – July 2002 HERS II (JAMA July 3) [Follow-up of HERS] – n = 2321 & 29 Breast Cancers – OR 1.08 (95% CI: 0.52 to 2.25) WHI (JAMA July 17) [Stopped early] – n= 16,608 & 290 Breast Cancers – OR 1.28 (95% CI: 1.01 to 1.62) HERS II & WHI –OR 1.26 (95% CI: 1.01 to 1.58) Revised Meta-Analysis of RCTs – WHI 68% weight –OR 1.20 (95% CI: 0.99 to 1.45)

Power Transform Prior – Results

Generalised Evidence Synthesis Modelling RCT & observational (3 types) evidence directly; –Hierarchical Models (Prevost et al, 2000;Sutton & Abrams, 2001) –Confidence Profiling (Eddy et al, 1990) Overcomes whether RCTs should form likelihood & observational studies prior

Generalised Evidence Synthesis Quasi RCTsCase-ControlRoutineRCTsCohort Beliefs Synthesis Decision Model UtilitiesCosts

Hierarchical Model

HRT: Hierarchical Model - Results IndependentHierarchical OR95% CrIOR95% CrI RCT to to 1.27 Cohort to to 1.13 CC-P to to 1.15 CC-H to to 1.36 Overall1.05 * 0.98 to to 1.24 * Ignores study-type

Hierarchical Model - Extensions Inclusion of empirical assessment of (differential) bias with uncertainty, i.e. distribution Bias Constraint, e.g. HRT

Discussion – 1 Direct vs Indirect use of non-RCT evidence –Direct: intervention effect, e.g. RR –Indirect: other model parameters, e.g. correlation between time points Allowing for bias/adjusting at study-level –IPD if aggregate patient-level covariates are important, e.g. age, prognostic score –Quality – better instruments for non-RCTs & sensitivity of results to instruments

Discussion – 2 Subjective prior beliefs regarding relative credibility (bias or relevance) of sources of evidence –Elicitation Bayesian methods provide … –A flexible framework to consider inclusion of all evidence, & … –which is explicit & transparent, BUT … –Require careful & critical application

References Deeks JJ et al. Evaluating non-randomised intervention studies. HTA 2003;7(27). Eddy DM et al. A Bayesian method for synthesizing evidence. The Confidence Profile Method. IJTAHC 1990;6(1): Ibrahim JG & Chen MH. Power prior distributions for regression models. Stat. Sci (1): Prevost TC et al. Hierarchical models in generalised synthesis of evidence: an example based on studies of breast cancer. Stat Med 2000;19: Sterne JAC et al. Statistical methods for assessing the influence of study characteristics on treatment effects in ‘meta-epidemiological’ research. Stat. Med. 2002;21: Spiegelhalter DJ, Abrams KR, Myles JP. Bayesian Approaches to Clinical Trials & Health-care Evaluation. London: Wiley, Sutton AJ & Abrams KR. Bayesian methods in meta-analysis and evidence synthesis. SMMR 2001;10(4):