The object of this book is to explain how to create a synthesis of complex biostratigraphic data, and how to extract from such a syn­ thesis a relative time scale based exclusively on the fossil content of sedimentary rocks. Such a time scale can be used to attribute relative ages to isolated fossil-bearing samples. From a practical point of view, the method described in this book will particularly interest paleontologists and geologists who must construct zonations and establish correlations on the basis of bio­ stratigraphic data that are both plentiful and apparently contra­ dictory. It is well known that the difficulties involved in constructing bio­ chronologic scales are largely due to the discontinuous nature of the fossil record. We know that the relationships between the first ap­ pearances (or disappearances) of different fossil species are rarely constant in stratigraphic sections that are distant from each other. It if often extremely difficult to discover datums or sets of species that are useful in making significant biochronologic correlations on a large scale. The theoretical model explained here (known as the Unitary As­ sociation Method) provides clear solutions to most of these problems. That method is purely deterministic, as opposed to statistical and probabilistic analytical techniques producing "average" ranges. We demonstrate in Chapter 15 why most of these techniques produce results which are usually not compatible with the original biostrati­ graphic observations (i.e., the taxonomic contents of the studied sam­ ples are not reproduced in the outputs).