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=='''Analytic studies'''==

=='''Analytic studies'''==

Analytic studies investigate possible associations between risk factors and disease by either comparing the risk factor exposure status in animals with disease to those without, or by comparing the occurrence of disease amongst 'exposed' animals to 'unexposed' animals. Although there are considerable difficulties in establishing causation of disease, the results of these studies can provide useful evidence regarding possible risk factors for disease. Although it is important to carefully consider the source population when undertaking these studies, the results may be able to be extrapolated out to a wider population than for descriptive studies. One example is that of the association between smoking and lung cancer - although a descriptive study of the prevalence of swine influenza amongst pigs in the UK is unlikely to be of much use in estimating the prevalence amongst pigs in the USA, an analytic study investigating risk factors for infection may (or may not) be similar. When considering whether or not results can be extrapolated out to a larger 'target' population, host, agent and environmental factors (such as breeds of pigs, strains of virus, farming systems and wildlife exposure) should be considered.

Analytic studies include '''cohort''', '''case-control''' and '''cross sectional''' studies, and may investigate possible associations between risk factors and disease by either comparing the risk factor exposure status in animals with disease to those without, or by comparing the occurrence of disease amongst 'exposed' animals to 'unexposed' animals. Although there are considerable difficulties in establishing causation of disease, the results of these studies can provide useful evidence regarding possible risk factors for disease. Although it is important to carefully consider the source population when undertaking these studies, the results may be able to be extrapolated out to a wider population than for descriptive studies. One example is that of the association between smoking and lung cancer - although a descriptive study of the prevalence of swine influenza amongst pigs in the UK is unlikely to be of much use in estimating the prevalence amongst pigs in the USA, an analytic study investigating risk factors for infection may (or may not) be similar. When considering whether or not results can be extrapolated out to a larger 'target' population, host, agent and environmental factors (such as breeds of pigs, strains of virus, farming systems and wildlife exposure) should be considered.

==References==

==References==