Measures of Population and Vital Statistics

•The objectives of measuring population, vital statistics and morbidity in the field of preventive medicine and public health are:

i. To calculate indices that measure levels of health and morbidity in a population.

ii. To compare natality. mortality or morbidity of different places, times, communities and professions. Com parison may also be useful as per age, sex and different socio-economic conditions.

iii. To monitor the progress made in the health and family welfare programmes.

iv. To fix up "priorities" in adoption of future health measures based on feed-back on the nature of the problem.

V. To determine the expectation or longevity of life at birth or at any particular age.

To determine the chances of survival or the proportion or percentage of survivors at any specific age following an intervention or therapy in a particular disease.

e.g. .. cancer.

• MEASURES OF POPULATION

• Mid-year Population (Mean population, population on July1st).Used in calculation of rates.

• The main source of population and its socio-demographical composition is census, carried out every 10 years.

• It forms the basis for projecting population data for any year.

• To calculate indices of health, the total events in a particular year such as births, deaths and morbidity of a defined geographical area form the numerator and the population exposed in that year on first July or mid-year population as popularly called, forms the denominator.

• This ratio may be multiplied by 100, 1000, 100,000, etc., to get the figure of convenient magnitude for easy comparison.

•  

• There are three methods for calculation of mid-year population:   

•  A. Natural Increase Method   • Natural increase implies the difference between

increase in population due to births and immigration and decrease due to deaths and emigration over a period of time say 1 year or 6 years.

• This can be added to the previous census population to get the mid-year population of a particular year up to which the net increase has been calculated.   

• B. Arithmetical Progression or AP Method   

Here it is assumed that there is equal increase each year and average increase per year is calculated in the intercensal period of 10 years such as between 1981 and 1991.  Population of any year or up to a particular month == last Census population + period in years and months after the last census month and year x yearly increase.  

• C. Geometrical Progression or GP Method

• It is easy to calculate with the help of log tables.

• MEASURES OF VITAL STATISTICS

• Absolute number of vital events such as births and deaths are converted into rates and ratios for comparison of vital statistics from place to place or year to year.

• Rate • Rate is a proportion with a defined denominator

termed as population. • The numerator is occurrence of an event which

is counted in the same population. Multiply by 100, 1000, 10,000, etc. as per magnification desired.

• When occurrence of an eventes rare such as cancer. multiply by 1.00.000.

• Rate differs from proportion in the matter of time. No time factor is involved in the proportion. The rate is a measure of the speed at which new events are occurring in a community. The rates, in general, are of two types crude and specific.

• i. Crude Rate- Crude rates are general rates calculated without paying regard to specific sections of the population. They measure the proportion between total events and the total population over a period of time.

• Crude rate

• Total number of events that occurred in a given geographical area during a given year__________ x 1000

Mid-year population of the geographIcal areas for the same period

• Specific Rate This rate is for specific group of the population such as for a particular age, sex, marital status, occupation, etc.

• Number of events which occurred among a specific group of the population of a given geographic area during a given year x 1000

• Mid-year population of the specific group of the population in the same geographic area during the same period

• Measures of Fertility. • Crude Birth Rate .• It is the most commonly used composite

index of increase in population by natality or child birth.

• The formula is: Annual crude live birth rate Number of live births which occurred among the population of a given geographic area during a given year x 1000

Mid- year population· of the same geographic area during the same year

• Note-'Llve' is distinct from 'total births' which include still births too.

• Fertility Rate

• This rate is related to women in the reproductive period hence it is better to measure the fertility of population in relation to the number of women aged 15 to 49 instead of the whole population.

• General fertility rate (GFR)

Number of live births in one year x 1,000

Number of women aged 15-49 years

• General marital fertility rate

In this, the denominator is only the married women in age group 15-49.

• Age specific fertility rate

Number of live births to mothers of a specified age group x 1000

Mid-year female population of same age group

• Total fertility rate

• From age specific fertility rates, a further measure called the total fertility rate (TFR) can be calculated.

• It gives an estimate of the average number of children born to a woman throughout her reproductive age as per' prevailing specific fertility rates.

• Measures of Morbidity

• Morbidity, illness, sickness or morbid condition mean deviation from a state of physical or mental well-being as a result of disease, injury or impairment.

• In a given population morbidity for a given time may be measured in terms of new cases (incidence) or In terms of new and old cases combined (prevalence).

• 1. Incidence Rates • Incidence tells how often an event occurs in a population

over a period of time such as' a week. a month. a year, etc .. e.g .. malaria or typhoid.

• i.e .. occurrence of new cases during a specified week, month or calendar year. etc.

• Spell of sickness means the duration of one illness only. • In a defined period, one may have two spells. e.g .. two

colds in 6 months. • The morbidity rates may be calculated for spells or

persons and the same may be mentioned in brackets. The denominator for calculation of rates is the mid-period popu lation exposed to risk.

• Incidence or inception rate (spells)

• Total number of new spells of illness during a defined period

= ------------------------------------------- x 1,000

• Population exposed to risk in the same period

• . Prevalence Rates Prevalence indicates how common is an event in a popula

tion. It is of two kinds:

a. Period prevalence b. It is Used to measure the frequency of an illness in

existence during a defined period (day, week, month, year, etc). It includes all the cases. in the defined period-old and new cases occurring 'during the same period.

• Period prevalence Total number of new and old cases found during a specified period

= -----------------------------------------------------------------X 1,000 Population exposed to risk at the same period

• b. Point prevalence This term is used to measure the num ber of cases of illness. new and old. existing at a particular point of time, such as at 2 PM on Monday. the 15th July. 1983.

• Point prevalence

Total number of new and old cases found at a particular point of time

= -------------------------------------------------x 1000 Population exposed to risk at the same point of time

• In a village with a population 5,000, 20 new cases of cholera occurred in January '1975 and 42 cases in 1976 in the months of June, July and August.

• Calculate monthly morbidity rate in 1975 and 1976.

• Monthly morbidity rate in 1975 = 20/5000x 1000 = 4.

• ie .. four cases per thousand per month and

• Monthly morbidity rate in 1976 42/3x5000 x 1000 = 2.8/ 1,000/month.

• There were 45 motor accidents in a town with a population of 69,000 in the year 1978. Calculate the incidence rate of accidents.

• Incidence rate = 45/69000 x 1000 = 0.65/ 1,000 per year

• Traffic police in a town or any large town makes use of this index to assess the benefit of their traffic control measures year by year.

• Tuberculosis survey was done in a town with 15,000 population. It was found that 27 cases were radiologically positive while 6 were sputum positive. Calculate the point prevalence rate of sputum positive cases at the time of survey.

• Prevalence rate of sputum positive cases

• = 6 x 1000 = 0. 4/ 1 000

15,000

• If 12 cases of diabetes mellitus were found in 15,000 population in a particular year. calculate the prevalence rate.

Prevalence rate= 12 x 1,000 =0.8/1000/year

15,000

• Morbidity rates may be calculated per 1,000. 10,000 or 1,00.000 depending upon the morbidity load in the com munity.

• Incidence rates are usually calculated for acute disease such as cholera. measles. diphtheria. burns. etc .. as their onset is sharply defined.

• Prevalence rates are determined for chronic illnesses such as leprosy, tuberculosis, filariasis, cancer, diabetes, hypertension etc. where the onset is very ill-defined. These rates measure the epidemiological situa tion. suggest priority in measures to be adopted and assess the progress made in the control of a disease.

• Measures of Mortality • i. Crude Death Rate • It is the most commonly used vital rate as it is easy to

compute. It measures the decline in population due to mortality in general, without specifying the factors that influence the deaths such as age-sex composition of population, occupation, causes of death, etc.

• Crude death rate Number of deaths which occurred among the population of

a given geographic area during a given year = ------------------------------------------------------------------ x1000

Mid-year total population of the given geographic area

during the same year • Crude death rate may also be written as:

• CDR = D/P x 1,000 where D stands for total deaths and

P for total population.

• Specific Death Rates • They are calculated to find the mortality experience in

different sections of the population such as infants, mothers, males and industrial workers or mortality due to different causes such as tetanus and tuberculosis.

• Age-sex death rates are high among the infants and the old and are lowest in the age group 10-14 years

• a. Age specific death rate

Number of deaths in the specified age group

= -----------------------------------------------------------X 1000 Mid-year population of the same age group

• Death rates under' one year of age or infant mortality rate

• They have special significance as they are very sensitive indices of health services and socio-economic advancement.

• Low rates indicate good MCH, obstetric. immunisation and health education services, sanitary environments and ade quate nutrition.

• . Still birth rate or late foetal death rate

Foetal deaths after 28 weeks of gestation

=------------------------------------------------------x1000

Live births + still births

• Perinatal mortality rate

• Still births and early neonatal deaths are attributed to similar causes, hence a single rate called perinatal mortality rate is calculated.

• It combines:

• _ foetal deaths after 28 weeks of gestation when the foetus becomes viable,

• - deaths during labour, and

• - deaths within seven days of neonatal life.

Perinatal mortality rate

Late foetal deaths (after 28 weeks or more) + deaths under one week

=---------------------------------------x 1000

Total births (live + still)

• C. Neonatal mortallty rate

Number of deaths upto 28 days of life

=--------------------------------------------------x 1,000

Number of live births

The causes of mortality in this period are antenatal. natal and postnatal. It may be divided into early neonatal up to one week. late neonatal after one week and up to the end of fourth week.

• D. Post-neonatal mortality or late infant mortality rate

Deaths after 28 days of life up to one year

=--------------------------------------------------x 1000 Live births

• In a town with a population of 50,000. there were 2000 births and 200 infant deaths in the year 1966.

• Of these 80 infants died in the first 28 days of life and 40 of them died in the first week of life. There were 110 still births in the same year.

• Calculate infant mortality rate, perinatal, neonatal and post-neonatal mortality rates.

Infant mortality rate 200/ 2000 x 1,000 = 100

PMR 110 + 40/ x 1,000 = 71.09 per thousand 2000+110

• Neonatal mortality rate 80/ 2,000 x 1,000 = 40 per thousand

• Post-neonatal mortality rate

200 - 80

= 2.000 x 1,000 = 60 per thousand.

• Sex specific death rate - female mortality rate is higher than that of males till the age of 35 due to neglect and repeated child-bearing.

Male death rate _ Male deaths x 1000

Mid-year population of males

• Age-sex specific death rate It gives the rate for specific age and sex e.g.

• Death rate of women in reproductive age group 15-49

=Female deaths in age group 15-49

- Female population of age group 15-49

• Maternal mortality rate

• This rate is important when applied to deaths in mothers owing to puerperal or maternal causes and is specifically known as maternal mortality rate.

• Number of deaths due

= to puerperal causes in females x 1,000 Number of live births

• Puerperal or maternal deaths include those due to complica tions of pregnancy. child-birth and puerperium.

• Cause-specific death rate Cause-specific death rates are computed for the total population.

• Numerator is often too small hence the rates may be calculated per 100.000 instead of per 1,000 to avoid small decimal figures.

• In country having a population of 430 million. 5,00,000 people died of tuberculosis. 50,000 of smallpox. 86,000 of tetanus. 3,00.000 of cancer and 6,10,000 of coronary heart disease in 1961. Calculate the cause specific mortality rates per 1,00,000

• Tuberculosis Death Rate 50.00.000 = 43.00.00.000 x 1.00.000 = 1162.8 per 1.00.000

• Smallpox DR 50.000 = 43.00.00.000 x 1.00.000 = 11.63 per 1.00.000

• Tetanus DR _ 86000

- 43.00.00.000 x 1.00.000 = 20 per 1.00.000

• Cancer DR _ 300000

- 43.00.00.000 x 1.00.000 = 69.77 per 1.00.000

• Coronary heart disease DR 6,10000

• = 43.00.00.000 x 1.00,000 = 141.66 per 1.00.000

• Proportional mortality rate• In this, death rates due to different causes are

compared by expressing them in percentages of the crude or total death rate.

• Suppose crude death rate is 20 and DR due to tuberculosis is 2. then the tuber culosis

DR = 2/20 x 100. i.e .. 10% of the total. • An increase in the proportional mortality may be

due to an increase in the number of deaths from that cause but it can also be due to a decrease in the total number of deaths from all causes. Similarly. decrease in the proportional mortality could be due to an increase in the total number of deaths.

• Case Fatality Rate (CFR)

It is the ratio of deaths due to a particular disease to the number of persons who suffered from the same disease.

• Number of deaths due a particular diseaseCFR =------------------------------------------------------- x 100 Number of cases of the same disease

• In a cholera epidemic caused by Asiatic cholera. Vibrio cholerae, 64 cases died out of 160 attacks while in typhoid before chloramphenicol. 60 died out of 200. Find the case fatality rates of both and compare .

64 x 100 • Cholera case fataIity rate = 160 = 40%

Typhoid case fatality rate = 60x100 = 30%

200• Thus, cholera had a fatality higher than typhoid. This

rate is also useful to compare the effect of treatment or any other procedure adopted to reduce the mortality such as chloram phenicol treatment in typhoid and preventive inoculation in cholera.

• Application of Rates and Ratios of Vital Indices

• Analysis of health statistics by computing various indices in the form of rates and ratios leads one to draw some con clusions. Interpretation of conclusions should be done care fully to make suitable recommendations to deal with the community health problems.

• Biased conclusions due to defect in registration. collection and compilation of data are very likely, When the records are not reliable, it is better to carry out sample surveys, following the rules for selection and inquiry strictly.

• Monitoring of illness in the Community

• Morbidity rates From the incidence or prevalence of dis eases, not only is the extent of illnesses in the country found but also conclusions are drawn about the role of environmental conditions.

• High incidence of cholera and gastroin testinal infections means poor water supply, drainage and disposal of refuse.

• High prevalence of tuberculosis means poor socio-economic conditions. Certain living conditions contribute to individual and community health.

• These can be assessed, such as food and nutrition. housing. place of work. recreation. social security. atmospheric pollution. etc. These are indicative of "level of living" and measure whether the desired standards of living have been achieved or not. They do not measure the actual health condition.

• Thus negative indices of community health are depended upon to draw conclusions about the health of a community. Of these indices, one should search for the index that can be the most comprehensive yardstick and then find the, specific ones.

Comprehensive Indicators that Measure Health • 1. Proportional mortality indicator • As defined before, it is obtained as a percentage

of the deaths at 50 years and above to the total deaths. There are more old people in developed countries and large number of deaths are there at the age of 50 years and over.

• In countries with poor health conditions and poor health services mortality is higher in younger children, in mothers due to specific causes and in other due to preventable and controllable communicable diseases.

• So, higher the percentage value of this index. the better is the health condition of the community.

• 2. Expectation of life

• Longevity of life worked out by the life table method, it is also a comprehensive measure of com munity health and summarises the mortality experience at all ages of life.

• In all developed countries,women live longer on an average by 5 years or more.

• In USA expectation of life at birth was 68.70 in males and 76.50 in females,the same ratio in Thailand was 65.40 and 60.80.

• Number of old people above 60 years was 5.1 % and in 1981 it was 5.6%.

• 3. Number of females • It is always higher than males in socio-

economically advanced countries while in the under developed countries, number of males is higher than females due to better health care of males.

• 4. Crude death rate This is usually the index available but should be made use of. only after adjusted or standardised rates are worked out. It is useful for short-term comparisons.

• Morbidity data collected in specific surveys can be a good indicator of comprehensive or specific health services.

• Nutritional status, absenteeism due to sickness in schools and industries and development of children from 1-4 years may be useful guides in the assessment of community health.