Book excerpt: The Health State Function of a Population This book will inspire colleagues in applying, developing and expanding the theoretical and practical issues related to the health state of the population and to improve forecasts related to the life expectancy and the healthy life span. The Second Edition of the book includes four more chapters presenting very important theoretical and applied work. The book deals with the theory related to the health state of a population and the introduced health state function. The book presents and applies the stochastic modeling techniques and the first exit time theory in demography along with the healthy life expectancy estimates and a derivation and classification of the human development stages. The data fitting techniques and the related programs are also presented. Many new and old terms are explored and quantitatively estimated especially the health state or the “vitality” of a population, the Deterioration and the related function and the Healthy Life Expectancy. A method for estimating the total loss of healthy life years with applications and comparisons for the healthy life expectancy in UK, Scotland, Sweden, and in the US States is added. A new Quantitative Method for Estimating the Human Development Stages based on the Health State Function Theory and the Resulting Deterioration Process is also included. The book is addressed to demographers, actuaries, statisticians, applied mathematicians, sociologists, psychologists, economists, health scientists, biologists, policy makers and scientists and practitioners of very many fields. Attention was given to prepare the material for readers from multidisciplinary fields thus including the appropriate formulas and mathematical typos along with many applications from computer programs in Excel. Read this book and share with us a fascinating exploration on life table data and the underlying information on human health state.