Why do surveillance?
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So why should we do surveillance?
So part of the answer to the question "When to do surveillance?" is if you need to do any of these things. In most emergencies, you need to do all of these things, so surveillance is a very high priority early in the emergency.
Surveillance can often produce information which is more timely than information gathered by other methods, such as surveys. It can also allow you to monitor trends more easily.