The number of persons in prison on a given day or in prison on average during a year differs significantly from the number of persons imprisoned (sent to prison) during a year. The first statistic measures a stock, while the second statistic measures a flow. The first statistic relates to prevalence, while the second statistic relates to incidence.

Consider, for example, a society that has just one prison cell. Suppose that each day, a person in prison is released and a different person is confined in prison. The average number of persons in prison during a year is one. The number of persons imprisoned during a year is 365. Alternatively, suppose one person was imprisoned on the first day of the year and remains in prison throughout the year. The average number of persons in prison remains one, but the number of persons imprisoned during the year is also one.

The number of persons who experience pre-trial (pre-disposition) incarceration or incarceration for less serious offenses is typically much higher than the number of persons incarcerated on a given day. In the U.S., jails hold persons incarcerated before either their cases as dismissed or they plead guilty in a plea-bargain (trials are very rare). Jails also hold persons incarcerated for sentences typically not exceeding a year. On a given day, about 650,000 persons are incarcerated in U.S. jails. In a year, more than ten million persons spend at least a single night in jail.

The number of persons in prison maps directly to the absence of persons. That statistic abstracts from the differential incidence of imprisonment. However, absence of persons is a common human sense. Its measure is consistent with every person’s equal human value. Moreover, absence in punishment can quantitatively encompass punishments of execution and banishment. Absence of persons in punishment provides a meaningful focus for understanding personal suffering.