A variable can be included in calculations in a BCA only if a monetary value can be assigned to it. An externality exists when there are costs or benefits associated with an action for which a monetary value is not readily assigned. Externalities are often referred to as 'non-marketed goods or services', as opposed to 'marketed goods' such as a drum of insecticide which can be purchased at a price determined by the market place. Determining the monetary value of an externality may be difficult because the factor being considered (e.g. maintenance of biodiversity) is not 'marketable'  or because links between the project actions and that factor (e.g. reductions in health problems if successful biological control reduces pesticide application) are unpredictable.

It is important to realise that although externalities are very difficult to evaluate, their inclusion or exclusion could radically alter the results of a BCA. Therefore it is important to at least check that the possible externalities for a biological control project are listed.  There are various ways in which economists attempt to evaluate non-marketed goods and/or services for inclusion in BCA. 

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