26 Mortality: Basic concepts and measures

Gautam Kshatriya and Gangaina Kameih

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CONTENTS

 

1.  Basic concepts of mortality

1.1. Introduction

1.2. Terminology used in mortality

1.3. Mortality determinants

2.  Mortality measures

2.1. Crude Death Rate (CDR)

2.2. Specific Death Rate (SDR)

2.3. Standardized Death Rate (STDR)

2.4. Infant Mortality Rate (IMR)

2.5. Neonatal mortality Rate (NMR)

Summary

 

Learning outcomes

After studying this module:

  • You shall be able to understand the concepts of mortality, the definition of the term used in mortality and the determinants of mortality
  • You will be able to describe, calculate and interpret the mortality indicators.

    1.  Basic concepts of mortality

 

1.1. Introduction

 

Mortality is the incidence of death in a country’s population. It is the continuous force of attrition that tends to reduce the population size. There are two fundamental differences between fertility and mortality situations prevailing in any country or society. Even in the remote past when life depended on the whims of nature, death rate compared to birth rate was small. Since birth rate was nearly at its biological limit to keep the population growing, death rate needed to be curtailed; this was not easy against the uncertainty in supply of food and nutrition. Other factors that could keep the death rate down-such as proper shelter, technological know-how and medicine needed to fight diseases and natural adversities were at their rudimentary stage. Population in the past, it is believed, did not show any sign of steady growth for thousands of years. That the population in modern times is still growing in most countries is a proof that death rates in population are still decreasing. In the middle stages, death rates fluctuated in all countries because of epidemic, famine or war. Secondly, fluctuation of death rate had generally remained small compared to birth rate. The mortality situation has steadily improved as the age of human civilization advanced. When the climate was good for agriculture, there was no shortage of food, no epidemic outburst and no war, death rates declined. No more do we need to depend totally on nature for our food; there has been a radical transformation in medical science and other technologies; death due to frequent wars between countries have come down; transport and communication network has improved remarkably to send medicines and other supplies to any ailing part of the world; there is some shelter for most people, which helps reduce exposure to natural calamities. There is no question of death rate shooting up suddenly. It is for this reason that we can fairly accurate predict the death rate, rate for population in the next five or ten years. But prediction of birth rate for a distant future has often failed. Death rate, unlike birth rate, responds quickly to prevailing social and economics standard in the country. That is, secular declining trend in mortality is more universal than decline in fertility trend. That is why fertility rate as a single base is less reliable and effective than mortality in population projection (Majumdar, 2010).

 

1.2. Terminology used in mortality

 

According to the United Nations and the World Health Organization (WHO, 1978), “death is the permanent disappearance of all evidence of life at any time after birth has taken place. It is the cessation of vital functions without capacity of resuscitation”. Further, it is to be noted that there are different stages of mortality and different terminology used in differentiating the various stages of death. Following are some of the definitions which are based on the 9th revision of international classification of Diseases by WHO (1978).

 

1)      Abortion: Expulsion or extraction from the uterus of a non-viable product of conception before 8 completed weeks from day one of the last menstrual period.

 

2)      Miscarriage: Loss of the product of conception at any time during the 8th to 12th weeks from day one of the last menstrual period.

 

3)      Foetal Death: Death prior to the complete expulsion or extraction from its mother, of a product of conception, irrespective of the duration of pregnancy.

 

4)      Still Birth: If on completion of 37 weeks the foetus is born dead, it is considered to be a still birth.

 

5)      Perinatal Mortality: The death of a live born within a few hours of birth.

 

6)      Neonatal Death: Death prior to completion of one month of life birth.

 

7)      Infant death: The death of a live born at any time from the moment of birth to the end of the first year of life.

 

8)      Early Childhood Death: Death up to four years of life.

 

9)      Late childhood Death: Death up to nine years of life.

 

1.3. Mortality determinants

 

The determinants of mortality include Supply of food and nutrition, shelter, technological know-how, medicine needed to fight diseases, epidemic, famine and war as it is also shown from the above introduction. Neonatal mortality is substantially higher among deliveries attended by untrained personnel, rather than those attended by trained health professionals, and the risk of infant mortality is lower for mothers of all age group if they received proper antenatal care (UNICEF, 1984, 1995; Pandey et al., 1998). The infant mortality deserves special attention in any study concerning mortality levels and indicators. It is so because the mortality in the early ages of life is more likely to influence the demographic structure of the society as compared to the mortality in the older ages (Waldron, 1987). In developing countries increased rate of infant and child mortality is one of the major deterrents in the adoption of small family size norms (Davis, 1945). The theoretical link between infant and child mortality and fertility is that increase in the child survival chances facilitate a corresponding decline in the propensity to “replace”, used by couples to ensure that they obtain desired family size. Social, economic, demographic and environmental factors, related to maternal and child health care are the known determinants of child mortality in general and infant mortality in particular. These determinants include ethnicity, parental education, place of residence, occupation, and household income, age at marriage, sex and birth order of the child, maternal age at birth, birth interval, availability of adequate obstetrical facilities and status of earlier siblings (Benjamin, 1965; Heer and Smith, 1968; Chandrasekhar, 1972; Caldwell, 1979; Farah and Preston, 1982; Trusell and Preston, 1982; Bourne and Walker, 1991). Mortality has been found to have a significant association with socio-economic development, programs aimed at controlling communicable diseases, health education, medical and health care and improvements in sanitation.

 

2. Mortality measures

 

There are several measures of mortality used by the demographer in which some are simple and some are complex, some are direct and some are indirect. They are crude death rate, specific death rates and standardized death rate. Apart from the crude death rate the most commonly used measures among the demographic Anthropologists are Infant mortality rate and Neonatal mortality rate.

 

2.1. Crude Death Rate (CDR)

 

Crude Death Rate (CDR) is the most frequently used and most easily calculated index of mortality in a population. It gives a general picture of mortality situation prevailing in the entire population. Crude Death Rate in a population or its segments often finds mention in all informal reports dealing with population. CDR sometimes represented by m, is calculated by the formula:

 

D refers to the total number of deaths occurring to all segments of the population and from all causes within a geographical (or social) boundary and in the same time frame, usually a calendar year.

 

CDR not only gives us a broad picture of the mortality situation prevailing in a population, it also gives us a broad picture of the mortality situation prevailing in a population, and it also gives us a probabilistic statement of dying of an individual within the time period. Though very handy, CDR has several limitations. If we will refer to the definition of the rate of a vital event assumes that the risk of dying for every individual in P is the same. This is an untenable assumption since the newly born babies, very young children and aged people die in larger numbers than those in the middle age groups. A mining labor has greater risk of life than a teacher or an office clerk. Women are more vulnerable in certain ages than men. There are considerable differences in death rates between regions and between communities, and even among the educated and illiterate people. For this reason it is not desirable to compare death rates of two countries or two states or regions of the same country by CDR. CDR for India in 2000 was 8.5 a figure very similar to those in many developed countries. Yet, India has an average expectancy of life of 63 years only when most developed countries have expectation of life at birth well over 75 years. Whereas the developed countries have 20 or less infant mortality (IMR), India still had 68 IMR in 2000. That is share of infants and young children in the deaths registered in a year is very high in India. In the developed countries the old age group contributes more significantly in the total number of deaths registered. CDR can reasonably well compare death rates at two not very distant points of time of the same region for the reason that age and sex composition of population generally do not change overnight.

 

When the population is relatively large, and there is fair amount of stability in economic, social and political front, the general composition of the population remains more or less at a stationary level. When CDRs for several consecutive years are known, a general index for those years can be obtained by taking simple arithmetic mean of CDRs of those years.

 

If D1  (2) refers to the number of those babies who die before attaining their first birth day and total number of babies who have not yet celebrated their first birth day,  number of babies die out of  number of babies who are between their first and second birth day, and so forth and so on, then are the death rates for the ages 1,2…(2) in that case CDR is calculated from the death rates at different ages in a population.

 

2.2. Specific Death Rates (SDR)

 

CDR speaks of the average mortality rate in the general population. But mortality rates vary between ages, between male and female, between urban and rural people, between various occupational groups, and etc CDR gives the mortality picture in a population at a glance but does not say if persons in some specific group in the population have the same probability of dying as in the total population. Specific death rate, SDR provides answers to this question. Generally, the population is classified into different segments by age, sex, residence, religion, occupation and other social and economic characteristics. We can compute mortality rate for all these segments separately. SDR is a ratio of the number of deaths in a specified segment in a specified space and time frame to the mid-year size of this segment in the said space and time frame multiplied by 1,000. SDR therefore is a CDR of a specific segment of the population.

     

 

With specific death rates, comparison between two population groups becomes more meaningful. Crude death rate for males in India was 8.9 against 8.1 of females in 2000. For getting an answer to the question why there was such a large difference, we may look for clues in sex-specific ASDR. SRS data show that IMRs for male and female babies are almost the same, around 71. But between age 1 and 3, the death rate among female babies is 1.5 times as large as that of male babies. Female death rates continue to be higher till about the age 24. Thereafter, male death rate overtakes female death rates. By the time age 50 is reached, male death rate is 1.64 times higher than that of female death rate. That is from 25 years onwards the probability of a woman dying is much smaller than that of a man. And this continues till the end of either’s career. But even with lower death rates after the 25th year, the losses of life experienced in the early years, especially in the age group 1-3 years, could not be compensated by higher survival rates at higher ages. That is why, sex ratio in the population in India continues against women.

 

We can compute cause specific death rate as other specific death rates are calculated. We have to classify deaths according to cause death. The subscript x in  in equation 4 would refer to ‘cause of death’ rather than age at death though both can be combined to give age-specific death rate for a particular cause. The denominator ( remains the same and refers to the population group for which causes of mortality were to be indexed. An illustration for calculation of ASDR appears in Table 1

 

Estimation of age-specific death rate is really difficult. Unless we know accurately the age of the deceased (we have to rely on the age reportedly by the doctor in his death certificate), errors are bound to creep in measures of ASDR. We do not have such a problem with CDR. Life Tables cannot be constructed without the accurate knowledge of age and sex-specific death rates.

 

Although age and sex-specific death rates are the most wanted death statistics, ASDRs are not without limitations. Though ASDRs are more scientific and practical and they can successfully be used for year-by-year comparison of two or more populations of different types, no unified picture about the mortality in a population can be had from ASDR. For this, we have to fall back on CDR or some such other measure.

 

2.3. Standardized Death Rate (STDR)

 

We have seen that ASDR do not tell us if population A has larger or smaller mortality rate compared to that of population B. One can compare two populations for their mortality level only if we could calculate some sort of an average taking into consideration the variability in different segments of the populations. The simplest such measure is CDR. CDR, in fact, is the combined average of the age- specific death rates in a population. If  are the average mortality for different age groups, the composite mean from these means is given by

 

Table 1 computation of ASDR, given total estimated 1999 population India

 

Note: The 1999 population has been estimated using compound interest law taking 1991 population as the base year population. The average annual growth was taken as 1.95 percent. Percentage of age group population to total populations (col.2) is that for the year 2000 (SRS). It is assumed that age distribution for the year 2000 is also valid for the year 1999. Col.4 is the SRS estimated percentage of death in each age group. CDR for 1999 is assumed to be the same as that for year 2000, i.e. 8.5. 8.5 multiplied by 984928511 (estimated 1999 is assumed to be the same as that for year 2000, i.e. recorded in 1999-2000. This is… the sum of total deaths in the population. The multiplied by the proportion of deaths in age-group (col.4/100) gives estimated number of deaths in each age- group. ASDR then is calculated by the usual formula. The column totals for estimated values do not agree with the numbers we had started with. The minor difference can be attributed to the approximate values for various proportions. The estimated ASDRs closely agree with SRS data for 2000 except in the last two groups.

 

This age-adjusted new crude death rate is referred to as standardized Death Rate, STDR. The selection of the standard population in calculation of STDR is vital. STDR will be heavily dependent upon the age composition in the standard population. Two different standards would invariably produce different results. When we intend to compare death rates for different regions or state, the age distribution of the whole country would be an ideal one to substitute the observed age distribution of the states or regions. However, one must insure that the substituted distribution is not radically different from the distributions which it substitutes. Ideally, Life Table stationary population distribution of the standard population is best in computing STDR.

 

The method we have discussed just now is a direct method for estimation of STDR. For each of the populations including the standard one, the age-specific death rate () and number of persons in each age () group (or their proportions in the total) must be known. There would be many instances where CDR and age distribution are known but not the ASDR. The method followed for estimation of STDR in such a situation is referred to as indirect method.

 

Indirect method estimation however, ASDR for the standard population must be known. Suppose  is the ASDR in the standard population and also suppose the ASDR in the standard population and also suppose the ASDR,  in population A is also known. We can compute 4 different types of average death rate:

 

(a)   . (CDR of population A)

(b)   . (STDR of population A)

(c)   . (STDR of population S taking population A as the standard)

(d)   . (CDR of population S)

 

If we closely look at the four computed average death rates, an approximate relation could be established between these four: (b) x (c) = (d). We can find out a close approximate value of STDR for population A from this:

(b) =

 

The ratios within parentheses in (9) are the adjustment factors. We show in the following Table 2 the computation procedure both by direct and indirect method. SRS data for 2000 has been used to compare general mortality rates in West Bengal and Tamil Nadu, the standard population begins India 2000.

 

Table 2 Steps in the calculation of standardized death rate

Converted into STDR, the difference in the general mortality rates in West Bengal and Tamil Nadu get reduced considerably, for about 1 point prior to standardization to only 0.16 after standardization by direct method and 0.37 when standardization is done by indirect method.

 

2.4. Infant Mortality Rate (IMR)

 

Infant mortality rate is defined as the number of deaths under the age of one year per 1000 live births for the year. In demography Infants are defined as “an exact age group, namely, age ‘zero’, or those children in the first year of life, who have not yet reached age one” (Barclay,1958). And it is important in the analysis of mortality because infant deaths account for a substantial number of deaths, especially in those countries where health conditions are poor (Bhende and Kanitkar, 2004). Infant mortality is one of the most frequently calculated measures of mortality (Arnold et al., 1998; Gupta and Baghel, 1999; Kertzer et al., 1999; Congdon et al., 2001). This measure is often used as indicator for the purposes of ascertaining levels of economic development and general levels of health among a population.

 

2.5. Neonatal Mortality Rate (NMR)

 

Neonatal mortality rate is defined as number of deaths during the first 28 completed days of life per 1000 live births in a given year or other period. Mortality during neonatal period is considered a good indicator of both maternal and newborn health care (WHO, 2011).

 

Based on the theoretical importance, the explanatory variables influencing mortality at the neonatal and early childhood ages are organized as follows:

 

1)      Maternal and bio-demographic factors: maternal education, age of the mother at giving birth, birth order of the child, birth interval, mother’s body mass index, sex of the child and weight of the children at birth;

 

2)      Health services utilization factors: full antenatal care, tetanus vaccination, place of delivery, and assistant at delivery;

 

3)      Household environmental factors: toilet facility, water facility, electricity, exposure to mass media;

 

4)      Socio-economic and regional variables: standard of living, caste, place of residence and region (Jalandhar and Arokiasamy, 2006).

 

Summary

 

Mortality is one of the factors that influence the statistic of any population, apart from affecting the health of the population on the individual basis. Study of mortality has become very significant for analyzing current demographic conditions; it also reveals the population’s health conditions. There are different stages of mortality and different terminology used in differentiating the various stages of death. The term includes abortion, miscarriage, foetal death, still birth, perinatal mortality, neonatal death, infant death, early childhood death, late childhood death. The determinants of mortality include Supply of food and nutrition, shelter, technological know-how, medicine needed to fight diseases, epidemic, famine and war. Social, economic, demographic and environmental factors, related to maternal and child health care are the known determinants of child mortality in general and infant mortality in particular. These determinants include ethnicity, parental education, place of residence, occupation, and household income, age at marriage, sex and birth order of the child, maternal age at birth, birth interval, and availability of adequate obstetrical facilities and status of earlier siblings. There are several measures of mortality used by the demographer in which some are simple and some are complex, some are direct and some are indirect. They are crude death rate, specific death rates and standardized death rate. Apart from the crude death rate the most commonly used measures among the Anthropological demographer are Infant mortality rate and Neonatal mortality rate.

you can view video on Mortality: Basic concepts and measures

 

References

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    Suggested readings

  • Budnik, A., & Liczbińska, G. (2006). Urban and rural differences in mortality and causes of death in historical Poland. American Journal of Physical Anthropology, 129(2), 294-304.
  • Johnston, F. E., & Kensinger, K. M. (1971). Fertility and mortality differentials and their implications for microevolutionary change among the Cashinahua. Human Biology, 356-364.
  • John Hopkins Bloonberg school of Public Health (2006). Mortality and morbidity, data sources for measuring mortality. Module 6a P: 1-53.
  • Maharatna, A. (2000). Fertility, mortality and gender bias among tribal population: an Indian perspective. Social Science & Medicine, 50(10), 1333-1351.
  • Weiss, K. M., & Wobst, H. M. (1973). Demographic models for anthropology. Memoirs of the society for American Archaeology, (27), i-186.

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