Measures of Health: Basic Measurements

Prof. Haroun O. Isah

MBBS, MPH, FWACP, FIMC, CMC, Cert. Stat. Epidemiol, Cert. APM

Provost

College of Medicine & Health Sciences

Bingham University, New Karu

Nasarawa State

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Tools

• Ratio, Proportion and Rate
• Ratio:
• A relationship between two quantities, normally expressed as the quotient of one divided by the other
• A comparison of two or more numbers that indicates their sizes in relation to each other
• It expresses the relationship between 2 numbers in the form of x:y or x/y x k
• Example: Ratio of male:female births in community A is 250:350 or 1:1.4
• Proportion:
• A part or share of the whole
• A specific type of ratio in which the numerator is included in or part of the denominator, and the resultant value is expressed as a percentage
• Proportion of all births that were male:

Male births/Male + Female Births = (250/250 +350) x 100 = 41.7%

Tools

• Rate
• This is the ratio between two related quantities in different units
• A special form of proportion that includes specification of time
• It’s the basic measure of disease occurrence because it’s the measure that most clearly expresses probability or risk of disease in a defined population over a specified period of time
• Defined as:
• # of events in a specified period/Population @ risk of these events in a specified period x k
• Example: number of deaths from HIV in a community in 2020 is 1500 among a population of 230,000. The rate of death due to HIV in that population or community is:
• 1500/230000 x 1000 = 6.5 per 1000

Types of Rate

• Crude Rate
• Summary rate based on actual number of events(births, deaths or disease) in a total population over a given time period
• It does not take into consideration potential factors that may alter or affect such events in an otherwise heterogeneous population
• CDR = (No. of live births to residents in an area in a calendar year/Population size in that area in same year) x 1000
• Specific Rate:
• Rate calculated for specific population attributes e.g. for sex, age, occupation and other demographic components i.e. for specific population groups
• For understanding epidemiologic aspects of an event and population dynamics
• ASDR (age 25 – 34) = No. of deaths among residents aged 25 – 34 in an area in a calendar year/Population size of those aged 25 – 34 in same area and in same period x 1000 (or 100,000)

Types of Rate

• Adjusted Rate
• Also referred to as standardized rate
• Summary rate that has undergone statistical transformation to permit/allow fair comparison between groups differing in some characteristics that may affect risk of disease
• Example is age-adjusted rate where it’s statistically transformed to remove the effect of age difference
• Two methods are commonly employed to achieve this:
• Direct Method: Age-specific rates observed in 2 or more study populations are applied to an arbitrarily chosen population of known age structure referred to as a “standard” population
• Indirect Method: Used in achieving comparison between 2 population, in one of which the age-specific rates are not known, or, if known, are excessively variable because of small number.
• In this method, the more stable rates of the larger population are applied to the population of the smaller study group
• The comparison of the number of expected deaths in the smaller population with the number actually observed yields what is known as SMR
• SMR= Observed Deaths/Expected Deaths
• It is possible to use SMR if the event of interest is occurrence of disease rather than death

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Content

• Measures of Morbidity
• Measures of Mortality
• Measures of Fertility

Measures of Morbidity

Introduction

• Definition of Morbidity:
• “Any departure, subjective or objective, from a state of physiological well-being”
• Sickness, illness, disability
• Morbidity is measured in 3 units
• Frequency - Incidence & prevalence
• Duration - Disability rate
• Severity – Case fatality rate
• Uses of Morbidity data:
• Describe the disease load in the community & assist in establishment of priorities.
• Provide more comprehensive, accurate & clinically relevant data essential for basic research
• Monitoring & Evaluation of disease control & prevention activities

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Incidence

• Definition: “The number of new cases occurring in a defined population during a specified period of time”
• Incidence Rate = (Number of new cases of specific disease during a given time period/Population at risk during that period) X 1000
• Example: Incidence Rate (IR) = (500/30,000) X 1000 = 16.7 per 1000 per year
• Incidence Rate refers:
• Only to New cases
• During a given period ( usually one year )
• Population at risk
• New spells or episodes of disease in a given period of time per 1000 population.
• Uses of incidence rate:
• Useful for taking action
• To control disease
• For research into etiology and pathogenesis, distribution of diseases, efficacy of preventive & therapeutic measures

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Incidence

• Tells us the rate at which new disease occurs in a defined, previously disease-free group of people
• The risk of acquiring the disease by those exposed to it
• To determine incidence, following are required consideration:
• Knowledge of study population health status
• Determine those diseased and not-diseased
• Time of onset
• Specification of numerator
• No. of conditions or cases ie No. of those who developed cold
• Specification of denominator
• No. of people at risk
• Period of observation
• In the formula:
• (No. of new cases of a disease in a defined time period/Population at risk during the defined period) x k

Incidence Rate: Types

A. Special Incidence rates

• Attack rate (Primary Attack Rate) = (No. of new cases of disease during specified time period)/Total population at risk during same period) x 100
• It is the percentage of population at risk , getting the disease

B. Secondary attack rate (SAR):

• The number of exposed persons developing the disease within the range of incubation period following exposure to a primary case
• Example: In family of 6 persons, consisting of 2 parents (immune) & 4 children, susceptible to chickenpox, one child develops chickenpox & after sometimes 2 children develop among three.
• SAR = 2 / 3 X 100 = 66 %

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Prevalence

• All current cases (old & new) existing at a given point of time, or over a period of time in a given population
• Definition: The total number of all individuals who have an attribute or disease at a particular time divided by the population at risk of having the attribute or disease at this point in time or midway through the period
• Two types – Point prevalence & Period prevalence
• Point Prevalence: the number of all current (old and new) cases of a disease at one point in time in relation to a defined population:
• = (Number of all current [old &new] cases of specific disease existing at a given time period/Estimated Population at the same point of time) = X 100
• Period prevalence: the number of all current (old and new) cases existing during a defined period or interval of time expressed in relation to a defined population
• = (Number of existing cases [old &new] cases of a specified disease during a given period of time interval/Estimated mid-interval Population at risk = X 100

Prevalence

Prevalence and Incidence

Determinants of Prevalence

• Prevalence is a function of the incidence of a disease and the duration of the disease

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Factors that Increase Prevalence Rate

Factors that Decrease Prevalence Rate

Uses of Prevalence

• Helps to estimate the magnitude of health problem/disease in the community
• Identify potential high risk population
• Useful for administrative & planning purposes

Errors in Disease Measurement

• Two basic kinds of errors: random (chance) and systematic (bias)
• Random: fluctuations around a true value because of sampling variability
• Systematic: any difference between the true value and that actually obtained that is the result of all causes other than sampling variability
• Sources of errors:
• Use of non-random samples of the target population
• Non-participation of members of the target group
• Observer variation
• Different response patterns
• Variations in perceptions of illness (illness behavior)
• Variations in availability of treatment resources

Measures of Mortality

Introduction

• Definition of Mortality
• “the permanent disappearance of all evidences of life at any time after birth” (WHO)
• A demographic event – Measure of average risk of dying of a person in the group during a time span
• Use of mortality rate
• Useful for projecting the future size of the population
• Identify population groups that are at high risk and in need of health service
• Indicatives of quality of life and expectation of life at birth
• Useful guides to planners
• Helpful to insurance companies

Introduction

• Sources of Mortality: There are two sources:
• Direct source:
• Registration of vital events
• Sample Registration Surveys
• National Family Health Surveys
• Indirect source:
• Age data of two consecutive censuses may be used to estimates death rates
• Demographic year book: U.N. Publication
• World Health Organization (WHO) also provides data

Measures of Mortality

• It is the quantitative and statistical devices to label the risk of mortality to which a population is exposed over a period of time.
• There a different measures of mortality:

1. Crude death rate (CDR): =( D/P) x 1000

• D= No. of deaths in a population during a given calendar year
• P= average number of persons living in the population during the year
• Merits
• Requires minimum data on mortality
• Easy to interpret
• Demerits
• Since the risk of death is not uniform for different segments of population (age, sex etc.)
• CDR is a crude measure and cant be used directly for comparing the levels of mortality in two or more countries

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Measures of Mortality

2. Age Specific Death Rates (ASDR): Mn-x= (Dn-x/Pn-x) × 1000, where:

• Dn-x = No. of death between n-x in the year.
• Pn-x = No. of persons aged between n-x in the year

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3. Infant Mortality Rate (IMR): No. of infants dying under one year of age in a year per thousand live birth in a given geographical region.

• IMR= (D₀/B) × 1000, where:
• D₀ : No. of infants who died before celebrating their first birthday.
• B : Total No. live births occurring in that year and geographical region.

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4. Child Mortality Rate (CMR): The total number of deaths of children aged 1-4 yrs. per 1000 population of the same age in a given year and geographical region.

• CMR = (No. of deaths of children aged 1-4 year in given year and region/total population aged 1 to 4 in the given year and a given region) × 1000

Measures of Mortality

5. Neonatal Mortality Rate (NMR): No. of infants dying within the first 4 weeks or up to 28 days of life per 1000 live birth in a year and geographical region.

• NMR = (Deaths of infants up to 4 weeks)/(No. of live births) = × 1000

6. Early Neonatal Mortality Rate (ENMR): No. of infant deaths during the first seven days of life per 1000 live births in a year and geographical region.

• ENMR = (Deaths of infants in the first week of life)/(No. of live births) × 1000

7. Perinatal Mortality Rate (PMR): No. of still births/late foetal deaths (after 28weeks of gestation) plus deaths within first week of life in a year and geographical region for 1000 births (live and still) in a year and region.

• PMR = (Foetal deaths after 28weeks of gestation + deaths of newborns within 7days)/ No. of live births during the same year = × 1000

8. Post Neonatal Mortality Rate (PNMR): Number of infants deaths after 28 days to less than 1yr (between 4 weeks to 52 weeks) of age per 1000 live births in a given year.

• PNMR = (No. of deaths of newborns between 4 weeks or less than 1yr old in a year)/No. of live births during the same yr) = 1000

Measures of Mortality

9. Maternal Mortality Ratio (MMR): No. of deaths of women while pregnant or within 42 days of termination of pregnancy from any cause related to pregnancy/childbearing and child birth per 100,000 live births in a given year.

• MMR = (Deaths of pregnant women and women after termination of pregnancy within 6 weeks from any cause related to pregnancy/No. of live births during the same year) = × 10⁵

10. Maternal Mortality Rate (MMRT): Number of maternal deaths while pregnant or within 42 days of termination of pregnancy from any cause related to pregnancy/childbearing and childbirth per100,000 women in reproductive ages 15-49.

• MMRT = (No. of maternal deaths of women in age 15-49/No. of women in age 15-49 in a given yr) = × 1000

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Measures of Mortality

• Foetal Death: Deaths prior to the complete expulsion or extraction from its mother of a product of conception at any time of pregnancy.

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• Still Birth: Death of foetus after completing 28 weeks and till the time of birth.

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• Incidence Rate (IR): Number of NEW cases of specified diseases occurring in a defined population during a specified period of time.
• IR = (No. of new cases of specified disease during a given period/Population at risk) = × 1000

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• Prevalence Rate (PR): Number of all current cases (old and new) of a disease at one point in time in relation to defined population.
• PR = (No. of new and old cases of specified disease existing at a given point in time/Estimated population at the same point in time) = × 1000

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Time Reference for Mortality in Childhood and Infancy

Exercise

Calculate the crude and Age specific death rates of the population from the following data

Common Causes of Death among Children

• Pneumonia and other lower respiratory diseases = 15%
• Preterm births and neonatal disorders = 12%
• Prematurity
• Birth Asphyxia
• Diarrheal diseases = 10%
• Congenital defects = 9%
• Infectious diseases = 45%
• Measles
• Malaria
• Neonatal Sepsis
• Other Infections
• Miscellaneous – 9%
• NCDs
• Injuries
• Nutritional deficiencies

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Limitations of Measures of Mortality

• The risk in a population may vary largely with various socio-economic and biological traits.
• The lack of reliable and requisite data presents serious problem sometimes of considerable magnitude.

Measures of Fertility

Measures of Fertility

• Fertility: is the actual birth performance of a group of women or the relative frequency with which birth occurs in total population or in the population exposed to fertility.
• Common measures of fertility:
• Crude Birth Rate (CBR)
• General Fertility Rate (GFR)
• Age Specific Fertility Rate (ASFR)
• General Marital Fertility Rate (GMFR)
• Age Specific Marital Fertility Rate (ASMFR)
• Total Marital Fertility Rate (TMFR)
• Total Fertility Rate (TFR)
• Gross Reproduction Rate (GRR)
• Net Reproduction Rate (NRR)
• Others

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Importance, Sources and Uses of Measures of Fertility

• Importance
• Fertility behavior of a given time gives shape to the future age-sex structure
• Hence, studying the fertility behavior has an implication on the overall welfare planning process
• Produce the alterations in the size of a population
• Sources of Data: There are three sources:
• Registration of vital events
• National Census/NPC
• 1981/1982 Nigeria Fertility survey (NFS)
• Federal Office of Statistics (FOS)
• Sample Registration System (SRS)/Sentinel Survey
• Nigeria Demographic and Health Survey (NDHS)
• 1965/1966 National Rural Demographic Sample Survey (NRDSS)
• 1980-82 National Population Bureau (NPB)
• Uses:
• Quantify the birth performance of a population over a period of time.
• Used to compare the fertility levels of a number of population, during a particular time interval
• To study time trend in fertility in entire population as well as differential fertility by various socio- economic, racial and ethnic groups.

Measures of Fertility

• Crude Birth Rate (CBR)
• Ratio of the total number of live birth to the average population ever lived during a given year and geographical area.

CBR = (B/P)× 1000, Where:

• B = Total No. of live births in a given year & area.
• P= Mid year population of the year
• Merits:
• Requires minimum data on fertility
• Easy to interpret
• Demerits:
• Cannot be used to compare the levels of fertility for any two populations because they may differ widely in their age-sex composition.
• It is not a fertility rate as it includes all the population either exposed or not exposed to the risk of child bearing.
• Age Specific Fertility Rate (ASFR)
• Age pattern of child bearing in any population is the best revealed by computing age specific fertility rates.
• It is the number of births per year per women in a given age group in a given year and geographical area.

Measures of Fertility

• General Fertility Rate (GFR)
• Simplest measure of fertility
• Defined as the number of births per year per thousand mid-year woman of the child bearing ages.
• Advantages:
• It includes the female population in their reproductive ages who are supposed to be exposed to the risk of giving birth.
• Generally used in population projection using component projection method.
• Total Fertility Rate (TFR)
• Number of children which a women of hypothetical cohort would bear during her lifetime if she were to bear children through out her life at the age-specific fertility rates for given year and if none of them dies before crossing the age of reproduction.

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Exercise

• The following table gives the population of a country for the year 1951, together with the estimated numbers of births and deaths based on a special vital statistics enquiry conducted in the country.
• Calculate:
• Crude Death Rate for
• Total population
• Males
• Females
• Crude Birth Rate for the total population
• General Fertility Rate

Answer to Exercise

• The following table gives the population of a country for the year 1951, together with the estimated numbers of births and deaths based on a special vital statistics enquiry conducted in the country.
• Calculate:
• Crude Death Rate for
• Total population: (Total Deaths/Total Population) x 1000 = (93828/6080020) x 1000 = 15.4 per 10⁵
• Males: (Total Male Deaths/Total Male Population) x 1000 = (48931/3055101) x 1000 = 16.02 per 10⁵
• Females: (Total Female Deaths/Total Female Population) x 1000 = (44897/3024919) x 1000 = 14.84 per 10⁵
• Crude Birth Rate for the total population: (Total Births/Total Population) x 1000 = (49719/6080020) = 8.18 per 10⁵
• General Fertility Rate: (Total Births/Total Population of Women of Reproductive Age) x 100 = 49719/1467580 x 100 = 3.39%

Other Measurements

Case Fatality Rate

• Describes the rate of death on account of a disease entity
• CFR (%) = No. of persons dying from a disease in a specific time period/Total No. with the disease x 100
• Measure of severity of a disease
• Can be used to measure benefits or efficacy of a treatment/new therapy
• Increased therapy efficiency – Decrease CFR
• Limitation: difficulty in differentiating deaths from other diseases
• Distinguishing between mortality rate and CFR
• In a population of 100,000 people of whom 20 contracted a disease in a year, 18 of the diseased cases died from it.
• Mortality Rate = 18/100,000 = 0.18%
• CFR = 18/20 X 100 = 90%
• Low mortality but high CFR
• In other words, it’s a rae disease but chance of death from it is high

Proportionate Mortality Rate

• Contribution to overall death by a specific disease
• Not actually a rate, but a ratio
• PMR = No. of deaths from a disease/Total No. of deaths recorded x 100
• PMR from CVD in Nigeria in 1999 (%):
• No. of deaths from CVD in Nigeria in 1999/Total deaths in Nigeria in 1999 x 100
• Answer will be the proportion of death caused by CVD
• If this proportion by CVD is eliminated, the number of deaths will be reduced by this proportion

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