The ratio of events having occurred in a population during a year to the number of persons in a population in the middle of the year. When the events are observed over a period shorter or longer than a year, their number is multiplied or divided by the appropriate factor so as to preserve the rate’s annual dimension. A rate may refer to all of the population (crude rate), or to an age or age group (specific rate) For certain phenomena, the rates are computed according to the period of time since the origin event: this is the case for duration-specific divorce rates. In this case, since the number of surviving marriages is unknown, the rate is calculated by determining the ratio of the number of divorces to the size of the initial marriage cohort.