This is Part I (Preliminaries) of the textbook Advising on research methods: A consultants companion, first published in print in 2008. It has been written for those who give advice on research methods in the social and behavioral sciences, including medicine and epidemiology. It aims to assist researchers in the field to produce better more accurate, valid, and reliable results. Herman J. Ader has been a statistical consultant for many years at VU University in Amsterdam first at the Psychology department, later at the VU Medical center. With Don Mellenbergh, he has given numerous courses on Methodological advising using the book as material. Gideon J. Mellenbergh is emeritus professor of Psychological Methods at the Department of Psychology, University of Amsterdam. He has a long-standing experience in methodological consultancy in different fields, such as clinical and health psychology, industrial and organizational psychology, educational research, social dentistry, and psychiatry. David J. Hand has been a Professor of Statistics at Imperial College, London, UK. He has wide-ranging consultancy experience to organizations ranging from banks, through pharmaceutical companies, to governments. He has published numerous, and was the founding editor of the journal of Statistics and Computing.