The Wodak and Cooney review had, from 11 studies of what they determined as demonstrating acceptable rigour, found 6 that were positive regarding the effectiveness of NSPs in preventing HIV, 3 that were negative and 2 inconclusive. However a review by Käll "et al." disagreed with the Wodak and Cooney review, reclassifying the studies on NSP effectiveness to 3 positive, 3 negative and 5 inconclusive. The US Institute of Medicine evaluated the conflicting evidence of both Drs Wodak and Käll in their Geneva session and concluded that although multicomponent HIV prevention programmes that include needle and syringe exchange reduced intermediate HIV risk behavior, evidence regarding the effect of needle and syringe exchange alone on HIV incidence was limited and inconclusive, given "myriad design and methodological issues noted in the majority of studies." Four studies that associated needle exchange with reduced HIV prevalence failed to establish a causal link, because they were designed as population studies rather than assessing individuals. Multi-component HIV prevention programmes that include NSE reduce drug-related HIV risk behaviors and enhance the impact of harm reduction services.