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Copyright ©Siswanto Agus Wilopo, [email protected] http://www.mkia-kr.ugm.ac.id or http://www.chnrl.net.id

Measures of Burden of Diseasesin Population

Siswanto Agus Wilopo

Department of Public HealthGadjah Mada University




Introduction

In its 1948 charter, the World Health Organization(WHO) defined health as “a state of completephysical, mental and social well-being and not merelythe absence of disease or infirmity.”

important ideological conceptualization, formostpractical purposes, objectives of health programsare more readily defined in terms of prevention ortreatment of disease.

to be able to diagnose and classify specific diseasesto the extent that such classification aids indetermining which health intervention programs wouldbe most useful.



Disease Frequency 3

Measures of DiseaseFrequency



Disease Frequency 4

Population

Group of people with a commoncharacteristic like age, race, sex



Disease Frequency 5

Two types of populations, based on whethermembership is permanent or transient:

• Fixed population: membership is permanentand defined by an event

Ex. Atomic bomb survivors

• Dynamic population: membership is transientand defined by being in or out of a "state.” Ex. Residents of the City of Boston



Disease Frequency 6

Want to quantify disease occurrence in a population

Measures of disease frequency should take into account:

Number of individuals affected with the disease

Size of source population

Length of time the population was followed

Disease Frequency



Disease Frequency 7

# New Cases Time Period PopulationCity A 58 1995 25,000

City B 35 1995-96 7,000

Annual "rate" of AIDSCity A = 58/25,000/1yr = 232/100,000/1 yr

City B= 35/7,000/2yrs = 17.5/7000/1 yr = 250/100,000/1 yr

Make it easy to compare rates by using same population unit(say, per 100,000 people) and time period (say, 1 year)

Ex. Hypothetical Frequency of AIDS in Two Cities



Disease Frequency 8

• Ratio: division of one number by another, numbersdon't have to be related• Proportion: numerator is subset of denominator,

often expressed as a percentage

• Rate: time is an intrinsic part of denominator, termis most misusedNeed to specify if measure represents events or

people

Three Classes of Mathematical

ParametersUsed to relate number cases of disease size ofpopulation and time



Disease Frequency 9

• P = Number of existing cases of disease /

Number in total population (at a point or during aperiod of time)• Ex. City A has 7000 people with arthritis on Jan

1 st , 1999• Population of City A = 70,000• Prevalence of arthritis on Jan 1 st = .10 or 10%

Prevalence : (P)Quantifies number of existing cases of disease in a population at a point or

during a period of time



Disease Frequency 10

Three key concepts :• New disease events, or for diseases that can

occur more than once, usually first occurrenceof disease

• Population at risk (candidate population) - can'thave disease already, should have relevantorgans

• Time must pass for a person to move fromhealth to disease

Incidence – Quantifies number of new cases of disease that develop in a population at risk during

a specified time period




Cumulative Incidence (Abbreviated CI)

Incidence Rate (Abbreviated IR)

Two Types of Incidence Measures




Cumulative incidence =

Number of new cases of diseaseNumber in candidate population over a

specified period of time

Two Types of Incidence Measures(Continued)




Cumulative incidence estimates the probabilityor risk that a person will develop diseaseDURING A SPECIFIED TIME.

Note that the candidate population is comprised ofpeople who are “at risk” of getting the disease

Used mainly for fixed populations because itsassumes that everyone is followed for the entiretime period

Two Types of Incidence Measures(Continued)




Example: Cumulative incidence of SuddenInfant Death Syndrome during first year of life

Population 1,000 livebirths

Cases of SIDS 10

Cumulative Incidence 10/1,000 or 1%over one year

• Note that all livebirths are „at risk‟



Age Cumulativerisk

Age Cumulativerisk

25 One in 19,608 60 One in 24

30 One in 2,525 65 One in 17

35 One in 622 70 One in 14

40 One in 217 75 One in 11

45 One in 93 80 One in 10

50 One in 50 85 One in 9

55 One in 33 0ver 85 One in 8

Age and Breast Cancer RiskAs shown by these cumulative incidence figures, the biggest single risk factor forbreast cancer is age.

SOURCE: National Cancer Institute SEER Cancer Statistics Review




• Cumulative incidence calculation assumes thatyou have followed the entire population for theentire follow-up period. For example, it assumesyou have followed all of the livebirths for one yearor until SIDS occurred.

• Often you can't follow everyone for entire timeperiod

• In a dynamic population, individuals enterpopulation over time, become lost, etc.

• So length of follow-up is not uniform for all

• Incidence rates do not make assumption ofcomplete follow-up




Incidence rate (IR) = # new cases of diseaseperson-time of observationin candidate population

This measure is a true rate because it directly

integrates time into the denominator.




Accrual of Person-Time

Jan Jan Jan1980 1990 1999

------------------x

------------------x

------------------------------------

10 Person-Years (PY)

10 PY

20 PY40 PY

Subject 1

Subject 2

Subject 3

X = outcome of interest, thus the incident rate is 2/40 PY




Some Ways to Accrue 100PY

• 100 people followed 1 year each = 100 py

• 10 people followed 10 years each= 100 py

• 50 people followed 1 year plus 25 peoplefollowed 2 years = 100 py

Time unit for person-time = year, month or dayPerson-time = person-year, person-month,person-day




• Followed 1,762 women ---> 30,324 py

• Average of 17 years of follow-up per woman

• Ascertained 61 cases of breast cancer

• Incidence rate = 61/30,324 py = .00201/y= 201/100,000 py (.00201 x 100,000p/100,000 p)

Ex. Cohort study of the risk of breast canceramong women with hyperthyroidism




Review of Dimensions

Prevalence = peoplepeople no dimension

Cumulative incidence = peoplepeople no dimension

Incidence rate = peoplepeople-time dimension is time –1




P / (1-P) = IR x DPrevalence depends on incidence rate and

duration of disease (duration lasts from onset ofdisease to its termination)

• If incidence is low but duration is long - prevalence

is relatively high• If incidence is high but duration is short -

prevalence is relatively low

Relationship between prevalence andincidence




– Steady state

– IR constant

– Distribution of durations constant

Note that if the prevalence of disease is low (lessthan 10%), the equation simplifies to P = IR x D

Conditions for equation to be true:




Figuring duration from prevalence andincidence

Lung cancer incidence rate = 45.9/100,000 py

Prevalence of lung cancer = 23/100,000

D = P = 23/100,000 pIR 45.9/100,000 py =0.5 years

Conclusion: Individuals with lung cancer survive 6 months fromdiagnosis to death




•Prevalence: administration, planning, someresearch

• Incidence: etiologic research (problems withprevalence since it combines IR and D),planning

Uses of Prevalence and Incidence

Measures




Common measures of diseasefrequency for public health

– Crude death (mortality) rate:

• Total number of deaths from all causes1,000 people For one year

(also cause-specific, age-specific, race-specificdeath rate)



Common measures of diseasefrequency for public health

Live-birth rate or crude birth rate :

total number of livebirths For one year1,000 people ( sometimes women of childbearing age )

Infant mortality rate :

# deaths of infants under 1 year of age For one year1,000 live-birth




Attack rate:

# cases of disease that develop during ________defined period_______ # in pop. at risk at start of period

(usually used for infectious disease outbreaks)




Case fatality rate:

# of deaths for a defined period of time# cases of disease

Survival rate:

# living cases for a defined period of time# cases of disease




Boston Globe Report on Celebrity Skiing Deaths

“Although skiing has inherent risks, it isn‟t moredangerous than other common activities. Thenationwide comparisons below do not reflectdiffering numbers of participants.”

Is this statement accurate? What are the numbers in the following charts? Incidence? Prevalence? or something else?

Skiing deaths (1996- 97 season)………………36

Skiing deaths (1984- 97 average)……..………34




Accidental Deaths: How does skiing compare to othersports? Remember to consider the missing denominatorswhen making your assessment.

Deaths in other sports

Parachuting…………………………………39

Scuba diving…………………………..…...104

Recreational boating (1996)………………716

Drownings in swimming, boatingand water sports (1995)………………...4,500 Bicycling (1996)……………………………800

SOURCE: News reports, National Ski Patrol



Accidental Deaths: How does skiing compare to otheractivities? Remember to consider the missingdenominators when making your assessment.

Other Deaths

Auto Accidents (1995)……………………42,000

Choking on food (1994) ……….…………..2,900

Hit by falling object (1994)…………………..800

Falls in Bathtub (1994)…………………….4,500

Struck by lightning (1996)…………………..800

SOURCE: News reports, National Ski Patrol




Four Issues Measuring Burden ofDiseases:

Reasons and Approaches for MeasuringHealth and DiseaseComposite Summary Measures ofPopulation HealthComparisons and Trends in DiseaseBurdenBurden of Risk Factors




Reasons and Approaches for MeasuringHealth and Disease

RationaleMeasuring Health and Disease

Counting Disease

Severity of Disease Mortality

Demographic and Epidemiologic Transitions

Morbidity and DisabilityMeasuring DisabilityData for Decisions




Assessing a Health System with Data:Historical Example

The health care dilemma in Ghana. The distribution of funds and personnel for primaryhealth care compared with costly hospital-based care is in inverse proportion to the numbers of

people that need to be reached. The health care pyramid for Ghana is upside down!




Source: United Nations Population Division, World Population Prospects: The 2002 Revision Population Database,




The Demographic and EpidemiologicTransitions

Changes in the pattern of disease proceed intwo steps:

demographic transition, when mortality frominfectious disease and undernutritiondecline with a marked drop in under-fivemortality plus a reduction in fertility;

epidemiologic transition, with a change indisease pattern.




Composite Summary Measures ofPopulation Health

Rationale for Composite MeasuresUses of Composite IndicatorsTypes of Composite Summary Measures

Fi 1 1 S i hi C f H h i l P l i Sh i h A f H l h




Figure 1-1 Survivorship Curve of a Hypothetical Population, Showing the Areas of HealthExpectancies. Source: C. J. L. Murray et al., Summary Measures of Population Health (Geneva, Switzerland: World Health Organization, 1999). Reprinted with permission.




Figure 1-3 : Different Patterns of Healthy Life Lost. Source: A. A. Hyder, G. Rotllant, and R. H.Morrow , “Measuring the Burden of Disease: Healthy Life Years,” 1998, American Journal of Public

Health, 88, pp. 196 – 202. Reprinted with permission.




Summary

Valuing Life: Social Value IssuesExpectation of Life Valuing Life Lived at Different Ages Valuing Future Life Compared with Present Life: Discounting Valuing Life for Its Economic and Social Productivity Valuing Equity in Relation to Efficiency




Data for Composite Measures

Types of Data Demographic Data.

Mortality.Morbidity.Variables

Checking Data Comparison of Total Numbers.Relationship Between Variables.Sensitivity Analysis.

Disease Groups: Classification Implementing a Burden of Disease Study




Figure 1-5 Burden of Disease in New Zealand,1996, by Age (a) and

Ethnicity (b and c). Source: New Zealand Ministry of Health. (2001).

Note: Total DALYs lost in New Zealand for 1996 = 500,000.

l b l f d




Global Assessments of Disease Burden

The Global Burden of Disease Mortality.Disability.Disease Burden.Age and Disease Distributions.

Other Ways Burden Can Be Measured Future Projections

Fig 1 9 Gl b l B d f Di 2000 b I




Figure 1-9 Global Burden of Disease, 2000, by IncomeLevel of Countries.

Source: World Health Organization. (2000)




Fi 1 10 Gl b l B d f Di 2000




Figure 1-10 Global Burden of Disease 2000by Disease Groups

Source: Murray, Lopez, Mathers et al. (2001).

Figure 1 11 Proportion of Disease Burden by Disease




Figure 1-11 Proportion of Disease Burden by DiseaseGroups in Selected Regions, 2000

Source: Murray, Lopez, Mathers et al. (2001).




B d f Ri k F t




Burden of Risk Factors

An analysis of risk factors that underlie manyimportant disease conditions can be useful forassisting policy decisions concerning healthpromotion and disease reduction interventions.Risk factors include an array of human behaviors,nutritional deficiencies and excesses, substanceabuse, and certain characteristics such as

hypertension




Figure 1 12 Linkages of risk factors with




Figure 1-12 Linkages of risk factors withhealth outcomes

The Burden of Selected Major Risk Factors




The Burden of Selected Major Risk Factors

focused on the quantification of trends in mortalityand, more recently, burden of disease.reliable and comparable analyses of risks tohealth, key for preventing disease and injury, havenot been quantified as well.Most analyses of the relation of risk factors tospecific diseases have been done in the context ofindividual risk factors, in limited settings, and withwide variations in the criteria for risk assessment.This has made comparisons of risk factors on apopulation health level difficult.




Attributable DALY (% of global DALY-Total 1.46 billion)




Figure 1-13 Mortality (a) and Burden of Disease (b) Due to Leading Global Risk Factors. Source: Ezzati, Lopez, Vander Hoorn. (2002).




Infectious and Non-Infectious Diseaseburden: epidemiologic polarization

I amgrowinginto a bigproblem

Conclusion




Conclusion

The first step in the pursuit of populationhealth improvement is the measurement ofhealth and disease.

Measurement is required to establish themagnitude of disease problems, definecausal factors, explore potential solutions,

and determine the impact of interventions.




Composite summary measures of populationhealth based on the amount of healthy life lostfrom disability and from death have beendeveloped to serve that purpose.Trends in disease burden provide important cluesto the success of ongoing health programs andthe need for development of new interventions.

Health systems across the world are greatlyaffected by changes in disease profiles andpopulation dynamics

Looking at the horizon with confidence



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