This approach is by far the most rigorous for exploring natural phenomena and is the preferred method of analysis for investigating the impact of biological control agents.

However, against this must be weighed the long-term costs, implicit in other approaches, of never having a definitive understanding of the workings of the system under study.

The basic requirement is to be able to apply biological control agents to weeds in such a way that their effects can be quantified. At the simplest level the hypothesis is that biological control agents will reduce the impact of the weed. The rates of change of population density, or in growth rate and/or survival of the weed, are compared between the treatment, in which the control agent is present on the weed, and the control, from which the control agent is absent. There are three basic methods to ensure that control agents are excluded from the experimental units to be used as controls: exclusion by insecticides, exclusion by cages, or controlled releases.

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