Terms such as "expanding Universe", "big bang", and "initial singularity", are nowadays part of our common language. The idea that the Universe we observe today originated from an enormous explosion (big bang) is now well known and widely accepted, at all levels, in modern popular culture. But what happens to the Universe before the big bang? And would it make any sense at all to ask such a question? In fact, recent progress in theoretical physics, and in particular in String Theory, suggests answers to the above questions, providing us with mathematical tools able in principle to reconstruct the history of the Universe even for times before the big bang.

This completely restructured, updated third edition of the volume first published in 1992 provides a general overview of the econometrics of panel data, both from a theoretical and from an applied viewpoint. Since the pioneering papers by Kuh (1959), Mundlak (1961), Hoch (1962), and Balestra and Nerlove (1966), the pooling of cross section and time series data has become an increasingly popular way of quantifying economic relationships. Each series provides information lacking in the other, so a combination of both leads to more accurate and reliable results than would be achievable by one type of series alone.

Our second book, Retention and Productivity Strategies for Nurse Managers, focused on leadership skills necessary for managers to decrease attrition and increase productivity. This book was directed at the nurse-manager rather than the student. The experience of completing research for the second book, coupled with our clinical observations, compelled us to incorporate more leadership content in our teaching and to write this book.