the speed or frequency with which an event or circumstance occurs per unit of time, population, or other standard of comparison. adjusted rate a fictitious summary rate statistically adjusted to remove the effect of a variable, such as age or sex, to permit unbiased comparison between groups having different compositions with respect to these variables. attack rate in the analysis of acute outbreaks of disease, the proportion of persons who are exposed to the disease during the outbreak who do become ill. basal metabolic rate an expression of the rate at which oxygen is utilized in a fasting subject at complete rest as a percentage of a value established as normal for such a subject. Abbreviated BMR. birth rate the number of live births in a geographic area in a defined period, usually one year, relative to some specified population. For the crude birth rate, it is the average total population or the midyear population in the area during the period. Specific birth rates for subsets of the population may also be calculated, for example, an age-specific birth rate is limited to the population of females of a defined age range.