Approach
The aquaculture authorities decide to use a survey to gather evidence of freedom, using a two-stage survey design (sampling villages at the first level, and ponds at the second). Laboratory testing of specimens from a large number of farms is not considered feasible, so a combined test system is developed to minimise the need for expensive laboratory tests.
The unit of observation and analysis is, in this case, the pond, rather than the individual animal. This means that the diagnostic is being made at the pond level (an infected pond or a non-infected pond) rather than at the animal level.
The survey is therefore a survey to demonstrate that no villages are infected (using a random sample of villages and making a village-level diagnostic). The test used to make a village-level diagnostic is, in fact, another survey, this time to demonstrate that no ponds in the village are affected. A test is then performed at the pond level (farmer observation followed, if necessary, by further laboratory testing).