For example, if the effects of a medicine are studied, and the subject is instructed not to drink alcohol, there is no guarantee that the subject followed the instructions, so field studies often sacrifice internal validity for external validity.
For fields like biology, geology and environmental science, this is not a problem, and the field experiment can be treated as a sound experimental practice, following the steps of the scientific method.
A major concern shared by all disciplines is the cost of field studies, as they tend to be very expensive.
For example, even a modestly sized research ship costs many thousands of dollars every day, so a long oceanographical research program can run into the millions of dollars.
Pilot studies are often used to test the feasibility of any long term or extensive research program before committing vast amounts of funds and resources. The changeable nature of the external environment and the often-prohibitive investment of time and money mean that field experiments are rarely replicable, so any generalization is always tenuous.
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