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# Demography -Introduction and Tools

The word demography has originated from two Greek words ‘Demos’ and ‘Graphos’. Demography is the scientific study of human populations, primarily with respect to their size, structure and changes therein. Population size effected by fertility, mortality and migration.

#### Sources of Population Data

Population data may be obtained from:
1.    Population censuses
–    Defacto
–    Dejure
2.    Demographic sample surveys
3.    Registration system and administrative statistics including:
•    Vital registration (birth, deaths and marriage)
•    Population registers
•    Service statistics (from health and family planning programs)
•    International migration statistics
4.    Qualitative data

## The Tools of Demography

#### 1. Count

The absolute number of a population or any demographic event occurring in a specified area in a specified time period.

#### 2. Rate:

The frequency of demographic events in population during a specified time period (usually a year) divided by the population “at risk” of the event occurring during that time period.
Rates tell how common it is for a given event to occur. Rates may be

• Crude rates
• Specific Rates
• Standardized rates

#### 3. Ratio:

The relation of one population subgroup to the total population or to another subgroup; that is, one subgroup divided by another.

#### 4. Proportion:

The relation of a population subgroup to the entire population; that is, a population subgroup divided by the entire population.

#### 5. Constant

An unchanging, arbitrary number (for example, 100 or 1000 or 100000) by which rates, ratios, or proportions can be multiplied to express these measures in a more understandable fashion. (K or Constant).

#### 6. Cohort measure

A statistic that measures events occurring to a COHORT (a group of people sharing a common demographic experience) who are observed through time. Cohorts may be:
–    Birth cohorts
–    Marriage cohorts
–    School cohorts

#### 7. Period Measure:

A statistic that measures events occurring to all or part of a population during one period of time“. It takes a snapshot” of a population, in effect.

## Occupational Health Hazards -Introduction and Types

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