Glossary of main concepts, terms and symbols

                used in the demographic studies.


(*) Amongst various sources, this glossary of terms has been prepared by the consultation of the

      following texts:

- Chiang, C.L. (1968), "Introduction to Stochastic Processes in Biostatistic", John Wiley, New York.

- Cox, P.R., (1970), "Demography", Cambridge University Press, Bentley House, London.

- Hageman, L.A. and Young, D.M., (1981), " Applied Iterative Methods", Academic Press, Inc.- San

                      Diego, California.

- IUSSP, (1982), "Multilingual Demographic Dictionary", Ordina Edition, LiŤge, Belgium.

- Keyfitz, N., (1968), " Introduction to the Mathematics of Population", Addison - Wesley, Reading,


- Pitchford, J.D., (1974), "Population in Economic Growth", North Holland Publishing, Amsterdam


- Shryock, H.S. and Siegel J.S. (1976), " The Methods and Materials of Demography", Academic Press,

                     New York

- The World Bank, (1998), "World Development Indicators", Washington, USA.

- United Nations, (1996), "Demographic Yearbook", New Yotk.

- United Nations Population Fund, (1993), "Readings in Population Research Methodology", Social

        Development Center Chicago, Illinois, USA.


Demographical and statistical terms                                                            Symbols


* A *


- abortion

is defined, with reference to the woman, as any interruption of pregnancy

before  28  weeeks  of  gestation  with  a dead foetus. There are two major

categories of  abortion: spontaneous and induced. Induced abortions are

those  initiated  by  deliberate  action  undertaken  with  the  intention  of

terminating pregnancy; all other abortions are considered as spontaneous.                              

- abortion rate

is a measure  of  the  frequency of abortion in a population during a given

period, usually a year.  Abortions  may  be  related to the total population

or to the number of women in the reproductive  ages, and may be specific

for age, parity or any other characteristic.

- abridged life table

is one in which most functions are  given  only  for  certain  pivotal  ages,

frequently spaced at five or ten-year intervals after infancy.

It  is  a life table  computed  by  age  intervals  of  5 or 10 years, instead of

single  years  of  age.  The youngest intervals are usually specified as age

0-1, 1-4. The oldest age interval is an open interval.

- absolute average difference                                                                            ISC

estimates the degree of fitting between observed and theoretical data.          

- abstinence

abstinence from coitus, particularly periodic abstinence is often included

among contraceptive or birth control method.

- adjusted rates

rates which have  been  standardized  for age and/or other compositional

characteristics in order to make them comparable with each other.

- age and duration-specific divorce rates

are given by the  number  of  divorces  of  women  (or men)  aged x to x+n

and married t to t+k years,  divided  by  the  persons  years  lived   by  the

corresponding married population.

- age-child ratio                                                                                             IV or ACR

defined as the ratio of the elderly (age 65 and older)  to the  population  in

in younger ages (under age 15).

- age dependency ratio                                                                                      ID

is the ratio of the young and the elderly to the adult population.

- age distribution of a population

when the distribution of a population is given by individual years of age.

- age groups

represents the age distribution of  a  population,  which  may be five-year

age groups, also called quinquennial age groups or broad age groups,

such as 0-19 years, 20-59 years, 60 years and over.

- age of maximum value of fertility rates.                                                       FXM

age corresponding to the maximum value of a given age-specific

fertility rates distribution.

- age heaping

tendency of some people to round their ages to numbers ending in zero

or five or to report certain ages instead of others. Also known as "age

preference" or "digit preference".

- age of maximum value of the resistance function                                          XM

age corresponding to the maximum value of resistance function.

- age-pattern of fertility

relative (proportionate) distribution of a set of age-specific fertility rates.

It espressed the relative contribution of each age group to total fertility.

- age-sex structure of a population

the number or proportion of the total population which falls in each

category of a cross-classification by sex and age (either single years or

age groupings).

- age-specific death (mortality rate), see Central death rate.

- age-specific divorce rate

is given by the number of divorces of women (or men) aged x to x+n

divided by the number of the person years lived by married women

(or men) between ages x and x+n.

The denominator may be also represented by the total number of the

person years lived at age x to x+n by women or men.

- age-specific fertility rate                                                                           f(x) or F(x)

rate calculated by relating births generated during a give period from

a  number  of  women  of  age  x  (or of age x,x+h)  to  the  quantity of

women living of age x (or in age x, x+h).

- age-specific first marriage rate

is given  by  the  number  of first marriages of women (or men) aged x

to x+n in a given year divided by the person years of women (or men)

aged x to x+n who are exposed to the "risk" of first marriage.

As second  kind,  the  denominator  may  be  represented by the total

 number of person years lived by men or women aged x to x+n.

- age-specific first net migration rate

is  computed  as  the  amount  of  net  migration  (difference  between

immigration and emigration) at a given  age  per  1000  of  the midyear

population at this age.

- age-specific probability of emigration

the annual  age-specific  probability  of emigration between exat age x

and x+1 is computed as the number of emigrants at a given age during

the year related to midyear population at age x, plus half of the deaths

during the year at age x.

- age-specific remarriage rate

is given by the total  number of remarriages for persons aged x to x+n,

divided  by  the  total  number of person years lived by divorced, plus

 the number of the widowed between age x to x+n.

- age standardization

a procedure of adjustment of crude rates ( birth,  death or other rates)

designed  to  reduce  the  effect of  differences  in age structure when

comparing rates.

- average annual rate of change

the  average  per  cent  increase  or  decrease  per  annum required to

account for observed numerical change in size between two dates.

- average family size

is the number of children for couple.

- average parity

is the mean number of children even born per woman.





Demographical and statistical terms                                                           Symbols


* B *


- balancing equation

addition of the net balance of the components of change (births, deaths

and migration)  between  two  dates  to the total population at the earlier

date in order to estimate the population  count  as of the later date. This

may be compared with a population count for the later date.

- basal body temperature method

refers to the contraceptive metod in which the woman keeps track of her

temperature to identify the safe fecond period..

- birth at term

birth which is non premature.

- birth cohort

a  sub-population  born  in  a particular year or set of years, the members

of  which  therefore are  of  the  same  age  range  and  pass  through  life

together Their progress may be charted by following them at successively

older ages in consecutive censuses or enumerations.

- birth intervals

time elapsed (months)  between  successive  live  births,  or  the intervals

between marriage and the first birth, between the first and the second bitrh;

and so on.

- birth order

the ordinal number of a given live birth in relation to all previous live birth

in relation to all previuos live births of the same woman.

- birth probabilities

the annual probability that a woman will have a live birth within a specified

year of age.

- birth rate                                                                                                        b(t)

rate  calculated  by  relating  the  number  of  live   births   observed   in   a

population  during  a  given  period  to  the  size  of the population during

the period. This rate is usually stated per 1,000, and the most usual

period is one year.

- birth spacing

is commonly used to refer to the deliberate efforts of couples to pospone

a birth.

- birth timing

is the interval between marriage and the first birth, and intervals between

successive births.


Demographical and statistical terms                                                            Symbols


* C *


- cause of death

the disease or event that caused death.

- cause-specific death rates

the  annual  number  of  deaths   from  a  given  cause  per  100000  mid-year

population .  Cause-specific  rates  are  reported for  sex-age  groups,  local

areas, and sometimes other categories.

- census

a complete  count  or  enumeration  of  a  population  conducted  under  the

auspices of some governmental authority. This term denotes that an attempt

was made to enumerate every member of the  population  concerned  and to

achieve complete coverage of the population. A micro census is limited to a

sample of the population, usually large in size, and belongs in the category

of sample surveys. Censuses or surveys are sometimes preceded by pre-tests

or pilot surveys.

- central death rate at age x                                                                  n m x ; m(x) ; M(x)

is the ratio of deaths between ages x and x+n to the mean population alive at

that age

- childbearing ages

the span wžthin which women are capable of bearing children, generally

taken to be from age 15 to age 49.

- childbirth

is the actual process of expulsion of the foetus. (see delivery)

- childlessness

refer to the state of a women, man or couple who have been so far infertile.

- child-women ratio

number of children aged 0-4 (or 5-9) per 1000 women of childbearing age

( f.e., 15-49) at the time they born those children.

- closed population

in which there is no migration either inwards or outwards and whose growth

is determined solely by fertility and mortality, and therefore depends entirely

on the difference between births and deaths.

- coding scheme

the coding scheme establishes a correspondence between an entry and its

translation into numeric or alphabetic codes.

- coefficient of determination                                                                     R2 or RR

estimates the fitting degree among observed data and theoretical data.

- cohort

denotes a group of persons who experience a certain event in a specific period

of time.

- cohort fertility

the fertility experienced over their life-time of a group of women or men who

form a birth or marriage cohort.

- cohort fertility rate

fertility  of  real  cohorts  of  women,  obtained  by  tracing them longitudinally

from year to year. Cohort rates may be age-specific or cumulative.

For completed  cohorts  which  have  passed  their 49th birthday, it is possible

to compute for each a general fertility rate, total fertility rate, and other summary


- cohort life table

records  the  actual  mortality  experience  of  a  particular  group   of individuals

(the cohort) from birth to the death of the last member of the group.

- completed fertility

is the cumulative  fertility  until  the  date  when  all  members of the cohort have

reached the end of the reproductive period.

- complete life-table

a life table  computed  for  every  single  year  of  age  from  birth  to  the  oldest

specified age.

- contraception

this  term  refers   to   measure   which  are  taken  in  order   to  prevent  sexual

intercourse or coitus from  resulting  in  conception, or to the conscious effort

of couples to avoid conception through  rhythm,  withdrawal, abstinence, male

and female sterilization,  or  use  of  contraceptives:  intrauterine  devise (IUD),

oral   contraceptives,   injectable   contraceptives,   condom,  spermicide,  and


- crude birth rate at time t                                                                       b(t) or Br

when all live births are related to the entire population or subpopulation.

- crude death rate at time t                                                                      d(t) or Dr

this is an annual rate and consists of the ratio of the annual number of deaths

 occurring during a calendar year per 1000 mid-year population.

- crude divorce rate

is given by the total number of the divorces in a year divided by the total

number of person years lived by the population, or per 1000 mid-year

population. Similar rates may be considered setting in the denominator

the number of persons above age 15, or the total number of exinting

marriages (identical to the number of married men or women ) exposed to

risk of divorce.

- crude emigration rate

represents the amount of emigration per 1000 of the mid-year population of

a country .

- crude first marriage rate

is given by the total number of marriages of women or men in a year, divided

by the number of person years lived by the single population of women or

men. We have a similar rate when in the denominator is the single population

of age groups that may legally marry (for convenience 15 is taken as minimum


- crude gross migration rate

represents the amount of emigration plus immigration per 1000 of the mid-year

population of a country .

- crude immigration rate

represents the amount of immigration per 1000 of the mid-year population of

a country .

- crude marriage rate

is the total number of marriages divided by the total number of person years

lived by the total mid-year population, or by the population aged 15 or more.

- crude net migration rate

represents the difference between immigration and emigration per 1000 of the

mid-year population of a country .

- crude rate of natural increase                                                                        r(t)

is the difference between the crude birth rate and the crude death rate.

- crude remarriage rate

represents the total number of remarriages in a year, divided by the sum of

widowed and divorced men or women exposed to the possibility of remarriage.

- cumulative fertility

it is obtained when the age-specific fertility rates are summed from the cohortís

beginning of exposure to risk until some later date.

- cumulative net fertility

represents the sum of the products of the fertility rates of the cohort by the

probability of survival of the women to successive aged .

- current life table

considers  the  mortality  experience  of  a  given population during one short

period of time.

- curve fitting

curve  fitting  consists  of  finding  a  suitable equation to represent the trend

of the examined data



Demographical and statistical terms                                                             Symbols


* D *


- death

is the permanent disappearance of all evidence of life at any time after live

birth has taken place ( post natal cessation of vital functions capability of

resuscitation ). This definition therefore excludes foetal deaths.

- deaths at age x                                                                                           D(x)

represents the number of deaths from age zero and exact age x observed in

the life table

- deaths at age exact x                                                                                 d(x)

are given by the first derivative of the life function

- deaths between ages x and age x+n                                                        d x,x+n

deaths calculated by the difference between survivors of the life table of

age x and of age x+n

- death rates by causes

are the annual number of deaths in each cause group reported for the year

per the corresponding mid-year population.

- delivery

is called the actual process of expulsion of the foetus.

- demographic model

consists of a theoretical construct representing the evolution of a population

(of individuals, couples,  families,  households, etc.) and its structure on the

basis of its initial state and the effect of various demographic variables (such

as fertility, fecondability, mortality, etc.). In a  static model  these  variables

remain constant; in a dynamic model these variables  are  allowed to change

over time.

- demographic transition

the  historical  shift  of  birth  and  death  rates  from  high  to  low  levels  in

population.  The  decline  of   mortality   usually   precedes  the   decline   in

fertility,  thus  resulting  in  rapid  population  growth  during  the  transition


- density of population

is the number of person  in  a  year  total  population  per  square  kilometre

of total surface area of a country.

- dependency ratio                                                                                          ID

persons aged 0-14 and over 65 of given sex per person 15-64 of given sex.

- developed (or more developed) countries

countries  that  have  higher  levels  of  per capita income, industrialization

and modernization.

- developing (or less developed) countries

countries that have  lower  levels of  per  capita  income,  industrialization,

and modernization

- direct methods

systems  of   linear  algebraic  equations  can  be  solved  either  by direct

methods or by iterative methods.  For  system  of  moderate  size,  the  use

of direct methods is often advantageous.

- distribution of population at age x                                                            C(x)

is the ratio of the population of age x, x+n to total population

- divorce

is a final dissolution of a marriage, that is, that separation of husband and

wife  which  confers  on  the  parties  the  right  to  remarriage  under  civil,

religious and/or other provisions, according to the laws of each country.

- dizigotics twins

are  due  to  the  simultaneous  fertilization  of  two  or  more  ova  and the

resulting children may be different sexes.

- Dwelling unit

is a statistical abstraction denoting housing accomodation appropriate for

occupation by one household.




Demographical and statistical terms                                                         Symbols


* E *


- elderly population

persons above the old age, term frequently used to define the period of life

during which most persons are retired

- endemic disease

is  one  that  permanently  affects substantial  segments  of  a population,

in contrast with an epidemic, which spreads and  then disappears  within

a fairly short time; when it appears  in  a  large  number  of  countries, it is

called pandemic.

- expectation of life at age x                                                                    e(x) or E(x)

it is the average number of years of life  for  males  and  females  if  they

continued to be subject to the same mortality experienced in the year (s)

to which these life expectancies refer.

- expectation of life at birth                                                                      e(0) of E(0)

represents the mean length of life of individuals who have been subjected

since birth to the mortality of the considered life-table

- estended family

is one in which contains  families  within  primary  family.  Son, daughters or

other relatives of the head who reside, with their own spouse and/or children,

in the household.

These "families within families" often are called "sub-families".



Demographical and statistical terms                                                              Symbols


* F *


- family

consists of two or more persons related to each  other by  blood, marriage, or

adoption. A residential family is  one  that  occupies  a  housing unit as living

quarters. In such a case, the household head is also the head of the family.

The collectivity of  related  persons  in  sometimes  called  a "primary" family.

- family demography

concerns  itself  with  the  structure  of families  and  household (i.e.,their size,

composition, and distribution) and the family-related events such as marriage

and  divorce,  that  alter  this  structure   through   their   number,  timing  and


- family life cycle

this term is frequently used to describe a sequence of life events from birth to

death, though its more precise meaning refers to a  sequence  of parenthood

stages over the life course, from birth of the children through  their departure

from the home to their own childbearing .

- fecundity

is   a   measure  of   the  maximum   number  of  children    which  a  woman  is

physically  capable  of  producing,  or  the  capacity  of  a  men , a women or a

couple to produce a live child.

- female fertility rates

are fertility rates (see) calculated for groups of women.

- fertility

is a measure which refers to the number of children borne by a woman.

- fertility histories

are accounts obtained for individual women of the important events in their

reproductive lives, such as marriage, pregnancy, births, infant deaths, etc.and

their dates. Fertility histories are often obtained retrospectively from survey.

- fertility lifetime

is the cumulative fertility until the date when all members of the cohort have

reached the end of the reproductive period.

- fertility rate

is a rate calculated by relating the number of live births observed in a population

or sub-population during a given period to a group of individuals of the same sex

in the riproductive ages. The denominator is commonly the mid-year population

in the stated period, but it may also be the number of years lived by the group

during the period, or the mean size of the group.

- foetal death

is death prior to the complete expulsion or extraction from its mother of a product

of conception , irrespective of the duration of pregnancy; the death is indicated by

the fact that after such separation the foetus does not breathe or show any other

evidence of life, such as beating of the heart, pulsation of the umbelical cord,

or definite movement of voluntary muscles. Late foetal deaths are those of twenty-

eight or more completed weeks of gestation.

- force of mortality or instantaneous rate of mortality                                  m (x)

see " instantaneous death rate".

- forward survival

procedure  for  estimating  the  age  distribution  at  some  later date by projecting

forward an observed  age  distribution.  The procedure uses survival ratios, often

obtained from model life table.  The  procedure  is  basically  a  form of population

projection without the introduction of new entrants (births) to the population.

- fraction of the last interval of age lived                                                   h(x) or H(x)

represents the part of the last year lived by those who die at each age.



Demographical and statistical terms                                                               Symbols


* G *


- general fertility rate

ratio of the number  of  live  births  in  a  perio d to the number of person-years 

lived by women of childbearing ages during the period.

- generation

has been given  a  precise  meaning  and  refers  to  a  group  of  persons  born

within a specified period of time, generally taken as a calendar year.

- gross reproduction rate (female)                                                               GRR

is  computed   similarly  to   net reproduction rate,   on   the   assumption  that

mortality before the end of the reproductive age is zero.



Demographical and statistical terms                                                              Symbols


* H *

- household

consists of  persons  who  reside  together ,  i.e.,  occupy  a  housing unity as

living  quarters.  Each   household   has  a  head,   who  usually   is  the  chief

breadwinner. Living quarters occupied by a single person is also a household.



Demographical and statistical terms                                                               Symbols


* I *


- infant mortality                                                                                         Q(0)

is  the  mortality  of  live  born  children  who have not yet reached their first


- infant mortality rates

are the  annual  number  of  deaths  of  infants under one year of age per 1000

live births in the same year.

- infecundity

is the lack  of  the  capacity  of  a  men,  a  woman or a couple to produce a live


- instantaneous death rate                                                                           m (x)

is  the  limit  of  the nqx values (or probability at age x of dying before reaching

age x+n) as n tends to zero

- intercensal estimates

relate to dates intermediate  to  two  or  more  censuses,  and take the results of

these censuses into account.

- iterative methods

Systems of linear algebraic  equations  can be solved either by direct methods

or by iterative.

Iterative  methods  are  used primarily  for  solving  large and complex problems

for which, because of  storage  and  arithmetic  requirements,  it  would  not  be

feasible or it would be less efficient to solve by a direct method.



Demographical and statistical terms                                                             Symbols


* J *



Demographical and statistical terms                                                              Symbols


* K *



Demographical and statistical terms Symbols


* L *


- late foetal deaths

are foetal deaths of 28 or more completed weeks of gestation. Foetal deaths

of unknown gestational age are included with those 28 or more weeks.

- late foetal-death ratios

are the annual number of late foetal-deaths per 1000 live births in the me year.

- legal abortion

when the abortion occurs in conformity with the laws of certain countries.

- legitimate birth

may be defined as one whose father and mother were married to one another

at the time of conception.  An   illegitimate   child  or  child born out wedlock

may be legitimized or legitimated by the subsequent marriage of its parents.

- legitimate birth rates

are  calculated  with  legitimate  births  in  the  numerator  and  the  currently

married female population in the denominator.

- life table

describes the course of  mortality  throughout  the  life cicle.  Genarally  a  life

table contains: the death rate among persons of a given age x; the probability

of dying within a given age interval; the number of survivors to a specific age

from an assumed initial number of births; the number of years lived

collectively by those survivors within the given age interval; the person years

lived by a hypothetical cohort from age x  on  onward;  and the expectation of

life of an individual of given age.

- life table survival ratios

these  ratios  are  derived  from  the  L column (stationary population) of the

life  table.  They  are  simply  the  ratio  of  the   number   of   persons   in   the

stationary   population   that   would  be   alive  at a certain age to the number

of persons alive at an earlier age.

- life-time abortion rate

is the sum of age-specific abortion rates and is a synthtic measure of abortion

 per woman or per 1000 women.

- live birth

 is the complete expulsion or extraction from its mother of a product of conception,

 irrespective of the duration of pregnancy, which after such separation breathes 

or shows any other evidence of life such as beating of the heart, pulsation of the

 umbilical cord, or definite movement  of  voluntary  muscles,  whether  or not the

 umbilical  cord  has  been cut or the placente is attached; each product of such a

 birth is considered live-born regardless of gestational age.

- logistic population

is  a  population  growing  in  accordance  with  the  logistic  law  of  growth,

ie.,  a  population  in  which  the  growth  rate  decreases  as a linear function

of the population  already  alive  and  which  will  tend  asymptotically to an

upper limit.

- long-term emigrants

are residents or  persons  who  have  resided  continuosly  in the country for

more  than  one  year,  who  are  departing  to  take  up  residence  abroad  for

more than one year.

- logit of the probability of dying                                                             LN[q(x)/p(x)]

is given by the logarithm of the rate between the probability of dying before

reaching  age  x  and  the  probability  of  surviving  at  age x.    See  also  the

meaning of symbols q(x) and p(x) under the letter P.




Demographical and statistical terms                                                              Symbols


* M *


- marriage

is the act, ceremony or process by which the relationship of husband and wife

is constituted. The legality of the union may be established by civil, religious,

or other means as recognized by the laws of each country.

- maternal mortality

according  to  ninth  revision of the International Classifications of Diseases,

"maternal death" is defined as the death af a woman while pregnant or within

42 days of termination of pregnancy, from any cause related to or aggravated

by the pregnancy or  its  management  but  not  from  accidental or incidental


- maternal mortality rates

are the annual number of maternal deaths divided by the live births in the same


- maternity

is used to denote a confinement resulting in the birth of one or more children.

- maximum resistance value at age xm                                                    r(xm) or R(xm)

represents  the   maximum   value   which   the   function   (or  distribution) of

resistance reached at age xm.

- mean age of a population                                                                              MAP

is the average age of all its members

- median age of a population                                                                            MDP

is the age which divides the population into two numerically equal groups

- mean age of childbearing

represents the mean age of the mothers at childbirth

- median length of life                                                                                       P o

is the age at which half the original cohort of births have died

- mid-year population estimate

refer to the de facto population on 1 july, or to the mean of two year-end

official estimates.

- migrant

an individual whose place of residence differs at the beginning and the

end of a certain interval of time.

- migration

the movement of people across a specified boundary for the purpose of

establishing  a  new  permanent  residence.   Divided  into  international

migration (migration between countries) and internal migration (migration

with a country).

- model life table

is a life table which is based only upon the generalization of empirical

relationships derived from a group of observed life tables.

- monozigotic twins

occur when one ovum splits after fertilization; the resulting children must

always be the same sex.

- mortality rate at age x (see Central death rate at age x)                            n m x ; n M x



Demographical and statistical terms                                                            Symbols


* N *


- natural fertility

age pattern of marital fertility observed in non-contraceptive populations

where reproductive behavior is not affected  by  the  number  of  children

already born.

- natural family planning methods

this term has been applied collectively to cover the rhythm method, the basal

body temperature method, and other techniques which attempt to identify

stages of the womanís ovulatory cycle.

- natural increase

number of births minus number of deaths during a year or other span of time.

In some cases, it may be negative.

- neonatal mortality rate

annual number of deaths to infants under 28 days of life per 100000 live

births during the year.

- net reproduction rate (female)                                                                      R(o)

is defined as the average number of live daughters that could be born to a

hypotetical female cohort which would be subjected to a set of current

age-specific fertility and mortality rates

- nuclear family

a nuclear family is one consisting of a couple and their children, if any.



Demographical and statistical terms                                                              Symbols


* O *

- open population

an open population is one where migration is a significant component of

population change, either in size or composition, or both.

- order-specific total fertility rate

is given by the summation of age-specific fertility rates order by order.

- order-specific fertility rate

number of live births of a given birth order (1,2,3, and so on) occurring in

a year, per 1000 women in the childbearing ages



Demographical and statistical terms                                                                 Symbols


* P *


- 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 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 so-called safe period of the menstrual


- population censuses

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


- 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                                    q(x) or Q(x)

represents the probability that an individual at birth will die before exact

age x

- probability at age x of dying before reaching age x+n                            nqx

represents the probability that an individual of exact age x will die before

exact age x+n

- probability of surviving from birth to age x                                       p(x)  or P(x)

is the complement to one of the probabilities of dying between age zero

and x

- probability at age x of surviving before reaching age x+n                      n p x

is the complement to one of the probabilities of dying between age x and


- probable length of life (see Median length of life)



Demographical and statistical terms                                                              Symbols


* Q *

- quasi stable population

is a formerly stable  population  with  constant  fertility  and  gradually

changing mortality.



Demographical and statistical terms                                                              Symbols


* R *


- rate computation

the crude birth, death, marriage, divorce etc. are generally the annual number

of each of these vital per 1000 mid-year population.

- rate of population growth                                                                         r or TA

the increase or decrease of a population in a period divided by the number of

person-years lived by the population during the same period.

- rate of natural increase at time t, or b(t)-d(t)                                             n(t)

the difference between the births and deaths occurring during by the number

of person-years lived by the  population  during  the  same  period.  This rate,

which specifically excludes changes resulting from migration, is the difference

between the crude birth rate and the crude death rate.

- replacement-level fertility

the level of fertility at which a cohort of women on the average are having only

enough daughters to replace themselves in the population.

By  definition,  replacement  level  is  equal to a net reproduction rate of 1.0. The

total fertility rate is also used to indicate replacement-level fertility.

Generally,   in   industrialized   countries,   a   TFR   (total   fertility rate) of 2.1 is

considered to be replacement level.

- reproductive histories

are accounts obtained for individual women of the  important  events   in their

reproductive lives, such as marriages, pregnancies,  births,  infant  deaths, etc.,

and their dates. Reproductive histories are often obtained retrospectively from


- resistance function                                                                                      r(x)

is the mathematical expression of the resistance data computed on the life-table.



Demographical and statistical terms                                                                Symbols


* S *


- sex ratio

is the ratio between males and females (or vice versa) of a population.

- sex ratio at birth

is the ratio between male births and female births (or vice versa) of a observed


- sex structure                                                                                                C(x)

of the population is measured by the ratio of the total number of one sex to the

total number of the population.

- short-term emigrants

are residents or persons who have resided continuosly in the country for more

than one year, who are departing to work outside the country for one year or less,

and their dependants including domestic employees. 

- skewness of a fertility distribution                                                               IAS

represents the degree of asymmetry of a function or a curve of the age-specific rate

of fertility

- spontaneous abortion

abortion following  a  non-induced  intra-uterine  death,  which  may has occurred

some time before expulsion.

- stable population                                                                                          SP(x)

a   population   with   an   unchanging   rate   of  growth  and  an  unchanging  age

composition,   because    age-specific    birth  and   death   rates   have   remained

constant over a sufficient period of time.

- standardized birth rates

to  compare  the  fertility  of  different  populations,  standardized  birth  rates  are

often  used  to  eliminate  the  effect  on  the  birth-rate  of  certain  differences  in

structure of the population (most commonly the age and sex structure).

- stationary population                                                                               L(x) or n L x

number  of  person-years  lived between  age x and x+n by an original cohort of

100,000  for  a  population   with  100,000  annual  births  and  deaths,  or  stable

population in which the intrinsic rate of natural increase is zero.

- sterility

is so called the lack of the capacity of a men, a woman or a  couple  to  produce

a live child. Among women we distinguish primary sterility where  the  women

ha never been able to have children, and a secundary sterility, which arise after

one or more children have been born

- survival rates                                                                                             h SR x

survival rates are calculated from the stationary population as the ratio of the 

number of survivors in an age interval to the number in the preceding interval.

- surviving function                                                                                  

is the mathematical expression of the survivors of a life-table.

- survivors                                                                                                    lx or S x

number of survivors at age x out of an original cohort of 100,000 births.



Demographical and statistical terms                                                               Symbols


* T *


- total divorce rate

is the summation of the age-specific divorce rate and  is  defined  for  men  or

women. This rate is an indicator of a synthetic cohort giving the total number

of divorces a men or a woman experiences on average throughout  life  based

on observed period rates.

- total numbers years                                                                                    T x   

is the number of person-years lived at age x and older 

- total reproduction rate (or total fertility rate)                                          TFR

is the summation of the series  of  age-specific  fertility  rates,  or the average

number of children  that would  be  born  alive  to  a  hypothetical  cohort  of

women if, throughout  their reproductive years, the age-specific rates for the

specified year remained unchanged.

- true rate of natural increase

is equal to the difference between the intrinsic birth rate and the intrinsic death




Demographical and statistical terms                                                            Symbols


* U *

- urbanization

growth in the proportion of a population living in urban areas.



Demographical and statistical terms                                                             Symbols


* V *


- variance (empirical)                                                                                    DS

represents the variance computed on observed female age-specific fertility


- variance (theoretical)                                                                                  VA

represents the variance computed by the fertility function

- value of maximum fertility rate at age FXM                                        F(FXM)

is the maximum of observed or theoretical values of age-specific fertility


- vital index

is the ratio of the number of births to the number of deaths during a period.



Demographical and statistical terms                                                              Symbols


* W *

- women years

is the number of years lived by a given number of women during a stated




Demographical and statistical terms                                                               Symbols


* X *




Demographical and statistical terms                                                                Symbols


* Y *




Demographical and statistical terms                                                                Symbols


* Z *

- zero population growth

is a situation in which a population is in equilibrium, with a growth rate of

zero, achieved when births plus immigration equal deaths plus emigration.







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