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== 6.7#h(0.6em)Statistical Literacy


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Prerequisites]


      Causation #linebreak() 
                 #linebreak()
]


    A low level of HDL have long been known to be a risk factor for heart disease. Taking niacin has been shown to increase HDL levels and has been recommended for patients with low levels of HDL. The assumption of this recommendation is that  niacin causes HDL to increase  thus causing a lower risk for heart disease.


    What experimental design involving niacin would test whether the relationship between HDL and heart disease is causal?


    
=== What do you think?


    You could randomly assign patients with low levels of HDL to a condition in which they received niacin or to one in which they did not. A finding that niacin increased HDL without decreasing heart disease would cast doubt on the causal relationship. This is exactly what was found in a study conducted by the NIH. See the description of the results


    #link("http://articles.latimes.com/2011/may/31/news/la-heb-niacin-cholesterol-20110531")[here]


    .