Sometimes a book merely rests unnoticed or waits unused or stands lifeless on a shelf – although authors usually hope for more. In the hands of readers, a book becomes many books; and even in the hands of a single reader, it can be a different book each time it is picked up anew – like the same river never stepped in twice. Readers bring to a book a personal history and understanding of a topic, guided by necessity and motivation. Some, for their own edification or on a teacher’s instruction, may have read this book from start to finish. Others may have skipped around a bit, looking for parts relevant to them. And still others may have used it simply for reference.

Whatever your previous history or current motivation – whether a novice seeking an overall understanding, a curious researcher seeking to expand your repertoire, a relatively experienced investigator wanting answers to specific questions, or even a fellow author examining how and how well we have addressed various topics – we hope this book has expanded your knowledge and understanding of sequential analysis and observational methods. Used appropriately, they can be cost effective and allow you to address research questions with unique fidelity. If you choose to use them, we hope this book will speed you in your work.