Donald T. (Lehigh University, USA) Campbell & Richard D. (Syracuse University, New York, USA) Schwartz 
Unobtrusive Measures [PDF ebook] 

Thirty-five years ago, the four authors of this book addressed the problems of validity in social science research. They were interested in new and unused methods for obtaining information. The original edition and an expanded version have often been cited as justification for using novel means to supplement, if not replace, conventional techniques, especially survey and archival research. Illustrations abound in this book. While the novelty of the illustrations will keep many a graduate student amused, the more serious purpose is to authorize and motivate ingenuity in obtaining information. Even more fundamental is the strategy of combining very different methods so that research results can, by triangulation, withstand ’threats to validity’ that so frequently invalidate single-measure, conventional research.