Epidemiological tools have proved effective in establishing major causes of diseases like cholera and lung cancer but have had problems with more subtle health issues , and several recent epidemiological results on medical treatments ( for example , on the effects of hormone replacement therapy ) have been refuted by later randomized controlled trials . Epidemiological tools have proved effective in establishing major causes of diseases like cholera and lung cancer , but have had problems with more subtle health issues , and several recent epidemiological results on medical treatments ( for example , on the effects of hormone replacement therapy ) have been refuted by later randomized controlled trials .