  Morphology The science of the forms and structures of organisms; the form and structure of a particular organism, organ, or part. From Dorland's Illustrated Medical Dictionary. Philadelphia: W. B. Saunders Co. (c) 1994. Reprinted by permission. Mortality rate, death rate An estimate of the proportion of a population that dies during a specified period. The numerator is the number of persons dying during the period; the denominator is the number in the population, usually estimated as the midyear population. The death rate in a population is generally calculated by the formula Number of deaths duringa specified period x 10n Number of persons at riskof dying during the period This rate is an estimate of the person-time death rate, i.e., the death rate per 10n person-years. If the rate is low, it is also a good estimate of the cumulative death rate. This rate is also called the crude death rate. Neuropathology The branch of medicine dealing with morphological and other aspects of disease of the nervous system. From Dorland's Illustrated Medical Dictionary. Philadelphia: W. B. Saunders Co. (c) 1994. Reprinted by permission. Pathology 1. That branch of medicine which treats of the essential nature of disease, especially of the structural and functional changes in tissues and organs of the body which cause or are caused by disease. 2. The structural and functional manifestations of disease. From Dorland's Illustrated Medical Dictionary. Philadelphia: W. B. Saunders Co. (c) 1994. Reprinted by permission. Person-time A measurement combining persons and time, used as a denominator in person-time incidence and mortality rates. It is the sum of individual units of time that the persons in the study population have been exposed to the condition of interest. A variant is person-distance, e.g., as in passenger-kilometers. The most frequently used person-time is person-years. With this approach, each subject contributes only as many years of observation to the population at risk as he is actually observed; if he leaves after 1 year, he contributes 1 person-year; if after 10, 10 person-years. The method can be used to measure incidence over extended and variable time periods. Population-based Pertaining to a general population defined by geopolitical boundaries; this population is the denominator and/or the sampling frame. Prevalence The number of events, e.g., instances of a given disease or other condition, in a given population at a designated time; sometimes used to mean PREVALENCE RATE. When used without qualification, the term usually refers to the situation at a specified point in time (point prevalence). Note that this is a number, not a rate. Prevalence, annual The total number of persons with the disease or attribute at any time during a year. An occasionally used index. It includes cases of the disease arising before but extending into or through the year as well as those having their inception during the year. Prevalence, lifetime The total number of persons known to have had the disease or attribute for at least part of their lives. Prevalence, period The total number of persons known to have had the disease or attribute at any time during a specified period. Prevalence, point The number of persons with a disease or an attribute at a specified point in time. Prevalence "rate" (ratio) The total number of all individuals who have an attribute or disease at a particular time (or during a particular period) divided by the population at risk of having the attribute or disease at this point in time or midway through the period. A problem may arise with calculating period prevalence rates because of the difficulty of defining the most appropriate denominator. This is a proportion, not a rate. See also PREVALENCE. Rate A rate is a measure of the frequency of occurrence of a phenomenon. In epidemiology, demography, and vital statistics, a rate is an expression of the frequency with which an event occurs in a defined population; the use of rates rather than raw numbers is essential for comparison of experience between populations at different time, different places, or among different classes of persons. The components of a rate are the numerator, the denominator, the specified time in which events occur, and usually a multiplier, a power of 10, which converts the rate from an awkward fraction or decimal to a whole number: Number of events in specified period Rate = x 10n Average population during the period All rates are ratios, calculated by dividing a numerator, e.g., the number of deaths, or newly occurring cases of a disease in a given period, by a denominator, e.g., the average population during that period. Some rates are proportions, i.e., the numerator is contained within the denominator. Rate has several different usages in epidemiology. 1. As a synonym for ratio, it refers to proportions as rates, as in the terns cumulative incidence rate, prevalence rate, survival rate (cf.Webster's Third New International Dictionary, which gives proportion and ratio as synonyms for rate). 2. In other situations, rate refers only to ratios representing relative changes (actual or potential) in two quantities. This accords with the OED, which gives "relative amount of variation" among its entries for rate. 3. Sometimes rate is further restricted to refer only to ratios representing changes over time. In this usage, prevalence rate would not be a "true" rate because it cannot be expressed in relation to units of time but only to a "point" in time; in contrast, the force of mortality or force of morbidity (hazard rate) is a "true" rate for it can be expressed as the number of cases developing per unit time, divided by the total size of the population at risk. Relative risk 1. The ratio of the RISK of disease or death among the exposed to the risk among the unexposed; this usage is synonymous with risk ratio. 2. Alternatively, the ratio of the cumulative incidence rate in the exposed to the cumulative incidence rate in the unexposed, i.e., the cumulative incidence ratio. 3. The term relative risk has also been used synonymously with odds ratio and, in some biostatistical articles, has been used for the ratio of forces of morbidity. The use of the term relative risk for several different quantities arises from the fact that for "rare" diseases (e.g., most cancers) all the quantities approximate one another. For common occurrences (e.g., neonatal mortality in infants under 1500-g birthweight), the approximations do not hold. Risk The probability that an event will occur, e.g., that an individual will become ill or die within a stated period of time or age. Also, a nontechnical term encompassing a variety of measures of the probability of a (generally) unfavorable outcome. Statistics The science and art of collecting, summarizing, and analyzing data that are subject to random variation. The term is also applied to the data themselves and to summarizations of the data. Statistical terms are defined by Kendall and Buckland.1 1 Kendall MG, Buckland WR. A Dictionary of Statistical Terms, 4th ed. London: Longman, 1982. Supratentorial Superior to the tentorium of the cerebellum. From Dorland's Illustrated Medical Dictionary. Philadelphia: W. B. Saunders Co. (c) 1994. Reprinted by permission. Survivorship study Use of a cohort LIFE TABLE to provide the probability that an event, such as death, will occur in successive intervals of time after diagnosis and, conversely, the probability of surviving each interval. The multiplication of these probabilities of survival for each time interval for those alive at the beginning of that interval yields a cumulative probability of surviving for the total period of study.