CORRESPONDENCE operate a strict non-prescribing policy. A significant proportion of our notifications were provided by â€oe¿ non-statutory― sources such as voluntary agencies and drug users themselves. Although multisource enumeration methods are expensive and time consuming, they demonstrate the independence of different sources of information. The value of indi vidual sources for continued monitoring varies from place to place and from time to time, according to patterns of local service

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... sion and the views of the users themselves about the value of the different agencies. Local knowledge is vital for making choices about which agencies reflect trends most accurately, and choices may need to be revised as patterns of service use change over time.