There is no agreed 'gold standard' to indicate the effectiveness of a prevention program. The highest criteria are set by the Cochrane and Campbell collaborations, the Blueprints for Violence Prevention project, the US Government Department of Education's What Works Clearinghouse and the Rand Corporation's Promising Practices network. A combination of an experimental evaluation – or randomised controlled trial – replicated in several locations and with sustained effects beyond the period of the intervention is generally viewed as a necessary precondition for a program to be designated as 'proven'. Evidence from several trials re-analysed as part of a systematic review is a necessary precondition for ratification by the Cochrane and Campbell Collaborations.