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Wildlife damage control programs, whether they be directed at rodents or other species, exist because of particular needs. The first criterion in evaluating any such program is: does the program meet an existing need, i.e. does it effectively reduce damage? To answer this question, it is necessary to define the need. Need can be described in terms of the extent and severity of damage caused by rodents, or potential damage (which may occur, if no action is taken). Actual damage is measured most accurately by an on-site survey or inspection. This may involve measurement of a resource loss (for example, reduction in alfalfa yield in fields infested by pocket gophers, or damage to insulation caused by house mice in swine confinement buildings), measurement of competition (for example, reduced performance of livestock on range inhabited by prairie dogs), or measurement of the incidence of a disease in a rodent population.