National Cancer Institute standards for adverse drug reactions: A five-category scale for assessing adverse drug reactions ranging from none (0), to mild (1), moderate (2), severe (3), life-threatening (4) and death (5). Both continuous variables, for example white blood count, and categorical variables, for example nausea, can be converted to this grading scale.

National Center for Health Statistics (NCHS): The principal health statistics agency of the USA, with responsibility for designing and maintaining a variety of general-purpose descriptive health surveys on a continuous basis and disseminating these data for widespread use. [NCHS, 1989, Vital and Health Statistics, Vol. 1, NCHS, Hyattsville, MD.]

National Institute for Health and Clinical Excellence (NICE): A government body responsible for making clinical recommendations and guidelines in practice in the United Kingdom. The recommendations made by this body often provide the basis of National Health Service Policy on what treatments it offers. [www.nice.org.uk]

National Institutes of Health (NIH): One of the world's foremost biomedical research centres and the federal focal point for biomedical research in the USA. [Statistics in Medicine, 1990, 9, 903–6.]

Natural history of disease: The course of a disease when left untreated or when treated with the standard therapy. [Transactions of the Royal Society of Tropical Medicine and Hygiene, 1958, 52, 152–68.]

Natural history studies: The use of data, often from hospital databases, to study the typical course of a disease, including the symptoms and patient characteristics that influence prognosis.