glossary * P *
Demographical and statistical terms  Symbols 
parity women are also classified by parity usually on the basis of the number of children born alive. A woman who has not borne any live children is called a zeroparity woman. A oneparity woman has borne one child but no more, and so on. 

parity progression ratios are fractions whose denominator is the number of women with n children, and whose numerator is the number of women with n+1 children. 

parityspecific birth probabilities the numerator consists of the number of order x+1 occurring during a period, and the denominator consists of the number of women of parity x at the beginning of the same period. 

perinatal mortality ratios are the annual number of late foetal deaths plus infant deaths within the first week of life per 1000 live births in the same year. 

perinatal period is the period defined from the 28^{th} weeek of gestation to the seventh day of life, or also from the 20^{th} week of gestation to the 28^{th} day of life. 

periodic abstinence when coitus is avoided during the period when the woman is believed to be fecund and takes place only during the socalled safe period of the menstrual cycle. 

population censuses are taken to obtain information about the state of the population at a given time. 

population forecast is a projection in which the assumptions are considered to yield a realistic picture of the probable future development of a population. 

population momentum is the tendency for population growth to continue beyond the time that replacementlevel fertility has been achieved because of the large and increasing size of cohorts of childbearing age and younger, resulting from higher fertility and/or falling mortality in preceding years. 

population projections are calculations which show the future development of a population when certain assumptions are made about the future course of population change, usually with respect to fertility, mortality and migration. 

population pyramid represents graphically a population’s age distribution as a histogram. That histogram shows the population by age and sex and is so named because of its pyramid shape. 

premature birth is a pregnancy ending before the normal period of gestation. 

primary sterility when the woman has never been able to have children. 

probability of dying between age zero and x represents the probability that an individual at birth will die before exact age x. 
q(x) or Q(x) 
probability at age x of dying before reaching age x+n represents the probability that an individual of exact age x will die before exact age x+n. 
^{ }_{n}^{ q }_{x} 
probability of surviving from birth to age x is the complement to one of the probabilities of dying between age zero and x. 
p(x) or P(x) 
probability at age x of surviving before reaching age x+n is the complement to one of the probabilities of dying between age x and x+n. 
_{n} ^{p}_{ x} 
probable length of life (see Median length of life) 
glossary main page