Hans Hoffmeister & Moyses Szklo 
Epidemiological Practices in Research on Small Effects [PDF ebook] 

Epidemiology faces its limits: the search for subtle links between diet, lifestyle, or environmental factors and disease is an unending source of fear — but often yields little certainty. Studies on weak associations — or small effects — often produce contradictory results which confuse the public. In this book, numerous contributions and illustrated examples show the effects of this problem, and sets out how future research should be approached in order to minimize the problems, thus producing clear results of significance.