Evaluation could be considered as a systematic determination of a subject's merit, worth and significance, usually using criteria governed by a set of standards. 

Evaluation can assist an organization, program, project or initiative to assess any aim, realisable concept/proposal, or any alternative, to help in decision-making; or to ascertain the degree of achievement or value in regard to the aim and objectives and results of any such action that has been completed.

We use it to establish the effectiveness towards the prevention, control and management of pest activity.  For example, how effective is rodent control on a farm if there is nothing to compare it against? 

A suitable suggestion maybe that if a site does not have rodents then the plan is effective - possibly true. However if that property is continually using bait to control activity then is the plan actually working or are baits simply fire-fighting a longer term infestation. Add to that, what happens if tighter restrictions are then placed on the continual use of rodenticides?