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Dear James - you cannot add relative risk reductions. If you start with a patient with an absolute CHD risk of, say 30% in 5 yrs (very high), and reduce their cholesterol, then your risk will fall by about one-third (or 33% in relative terms) down to about 20% in 5 years. Then if you treat their hypertension you reduce their risk from 20% in 5 yrs by one-fifth( 20% in relative terms) down to 15% in 5 years etc. So you keep applying a relative risk reduction to an ever decreasing absolute CHD risk. Rod Jackson >The last couple of messages have alluded to the issue of independent risk >factors and if the benefit of treating more than one risk factor is >additive. > >To date, as far as I am aware, all the multiple risk factor intervention >trials have shown no clinical benefit - Systematic review of randomised >controlled trials of multiple risk factor interventions for preventing >coronary heart disease - BMJ 1997;314:1666 - 7 June > >In addition, if you reduce your cholesterol the chance of CHD decreases by >30%; if you treat your hypertension you reduce your chance of CHD by 20% and >if you treat your diabetes you reduce it by 25% and if you stop smoking you >reduce your chance by 40%. These numbers add up to 115% which is impossible >unless you are able to decrease the chance of CHD in the person standing >next to you. > >In this regard, I have a couple of questions for the group: > >1) If one reduces so called independent risk factor why does that not >produce additive benefits? > >2) In cohort studies where risk factors have been shown to be independent by >covariance analyses - does it mean that they should produce additive effects >when applied to patients with multiple risks. They appear not to, so why >not? Are they really completely independent or just different enough to >produce a statistical difference? > >3) When covariance analyses are done on data from RCT's to show independent >effects does that mean that these interventions should produce additive >effects? > >Any help to clarify these issues would be appreciated. > > >Dr James McCormack >Associate Professor >Faculty of Pharmaceutical Sciences >University of British Columbia >Vancouver, B.C. >604) 822-1710 >[log in to unmask] Rodney Jackson MBChB PhD FAFPHM Professor of Epidemiology Head of Department Dpt of Community Health Faculty of Medical & Health Sciences University of Auckland (Grafton Mews, 52-54 Grafton Rd) Private Bag 92019 Auckland, New Zealand Phone: +64 (0)9-3737599 ext 6343 Fax: +64 (0)9-3737494 e-mail: [log in to unmask] %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%