Causation confounding factors

Other health consequences of cadmium exposure, such as anemia, liver disease, nerve or brain damage, have been suggested, but the causation is less straightforward in these cases, probably because these conditions may depend on a variety of other confounding factors. 

There are several reasons why more than 100 independent studies failed to find a significant association between visual acuity and driving. The first is that crashes are caused by multiple factors (see Chapter 17 on crash causation), and impaired vision may be confounded by other variables (such as age and co-morbidity). For example, on the one hand the people with the... 

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