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 zero-parity woman. A one-parity 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.
 
parity-specific 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 28th weeek of gestation to the seventh day of life, or also from the 20th week of gestation to the 28th 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 so-called 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 replacement-level 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 populationís 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.
 nx
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)

 

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