basic measurements in epidemiology

Basic measurements in epidemiology

Dr. S. A. Rizwan, M.D.,Asst. Professor,

Department of Community Medicine,VMCHRI, Madurai

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Learning objectives

• At the end of this lecture you sh be able

• List the various tools used for measurement• List the various measures of death• List the various measures of disease

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Recap

• Define epidemiology• Uses of epidemiology

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Measurements used in epidemiology

• Measurement of mortality• Measurement of morbidity• Measurement of disability• Measurement of natality• Measurement of disease attributes• Measurement of health care services• Measurement of the risk factors• Measurement of demographic variables

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TOOLS OF MEASUREMENT

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Tools of measurement

• Proportion• Rate • Ratio

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Proportion

• Usually expressed as a percentage %– Numerator (which is part of denominator)– Denominator– Multiplier– No time factor

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Proportion – exampleWhat proportion of this class are Vijay fans?

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Proportion – exampleWhat proportion of this class are Ajith fans?

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Proportion – ‘real’ example

• What proportion of the population is suffering from diabetes?

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Rate

Contains• Numerator (which is part of denominator)• Denominator• Multiplier• Time period

• Usually expressed per 100 / per 1000 population• It has a time dimension, whereas a PROPORTION

does not

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Rate – example

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Rate – example

• Death rate = X 1000

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Ratio

Contains• Numerator (not part of denominator)• Denominator

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Ratio – example

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Ratio – example

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Ratio – example

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Summary

• Epidemiology uses 3 main tools of measurement– Proportion– Rate – Ratio

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MEASUREMENT OF MORTALITY

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Measurement of mortality

• Crude death rate• Specific death rate• Proportional mortality rate

• Case fatality rate

• Survival rate• Adjusted/standardized rates

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International death certificate

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Crude Death Rate

X 1000

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Total population in Madurai in 2015

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No. of deaths in Madurai in 2015

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CDR in Madurai in 2015

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Specific death rate

• Cause specific– Deaths due to cholera

• Age specific– Infant deaths

• Sex specific– Maternal deaths

• Time specific– Weekly deaths

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Total population in Madurai in 2015

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No. of deaths in Madurai in 2015

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Cause of death – cholera

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Cholera specific death rate in Madurai in 2015

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Proportional mortality rate

= X 100

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Total population in Madurai in 2015

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No. of deaths in Madurai in 2015

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Total deaths

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Cause of death – cholera

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Proportional mortality rate of cholera

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Total population (MYP)

Total deaths – all causes

Cholera deaths

CDR

Specific DR

PMR

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Case fatality rate

CFR = X 100

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Total no. of cholera cases

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No. of people died due to cholera

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Total no. of cholera cases

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Survival rate

• Used in research studies• Like cancer therapy or survival

X 100

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What is standardization?

You: My salary has doubled this month

Ur friend: My salary has also doubled

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What is standardization?

¥. 1 Lakh = Rs. 10 Lakh¥. 2 Lakh = Rs. 20 Lakh

Gain = Rs. 10 Lakh

Rs. 1 LakhRs. 2 Lakh

Gain = Rs. 1 Lakh

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What is standardization?

Rs. 1 Lakh = $ 1,500 ¥. 1 Lakh = $ 15,000Rs. 2 Lakh = $ 3,000 ¥. 2 Lakh = $ 30,000Gain = $ 1,500 Gain = $ 15,000

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What is standardization?

You: My salary has doubled this month

Ur friend: My salary has also doubled

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Direct standardization

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Crude ratesMadurai Population Deaths Death rate per

10000-64 53,500 446 8.3

Chennai Population Deaths Death rate per 1000

0-64 92,000 850 9.2

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Age specific ratesMadurai Population Deaths Death rate per

10000-24 21,500 123 5.7

25-64 32,000 323 10.0

Chennai Population Deaths Death rate per 1000

0-24 32,000 150 4.625-64 60,000 700 11.6

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Standard population

Standard Population

0-24 156,000

25-64 45,000

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Standardization for MaduraiStandard Population Madurai

ratesExpected

deaths0-24 156,000 5.7 889

25-64 45,000 10.0 450

Total 201,000 1,339

Age Population Total deaths Standardized crude rate

0-64 201,000 1,339 6.6

∂

∂ ∂ ∂

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Standardization for ChennaiStandard Population Chennai

ratesExpected

deaths0-24 156,000 4.6 718

25-64 45,000 11.6 522

Total 201,000 1,240

Age Population Total deaths Standardized crude rate

0-64 201,000 1,240 6.1

∂

∂ ∂ ∂

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Standardized ratesMadurai Population Deaths CDR per

1000Std. DR

per 10000-64 53,500 446 8.3 6.6

Chennai Population Deaths CDR per 1000

Std. DR per 1000

0-64 92,000 850 9.2 6.1

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Indirect standardization

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Standardized mortality ratio

Age National death rate per 1000

25-34 3

35-44 5

45-54 8

55-64 25

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Standardized mortality ratio

Age Doctor population

Observed deaths

25-34 300 *35-44 400 *45-54 200 *55-64 100 *Total 1000 9.0

Crude death rate for doctors is 9.0 per 1000

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Standardized mortality ratioAge National

rateDoctor

populationObserved

deathsExpected

deaths25-34 3 300 * 0.935-44 5 400 * 2.045-54 8 200 * 1.655-64 25 100 * 2.5Total 1000 9.0 7.0

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Standardized mortality ratio

SMR = X 100

SMR = 9/7 X 100 = 129

It means doctors experience 29% more mortality than the general population

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Summary

• There are various measures of mortality• Each one has its own purpose and

disadvantages• Standardization is a method for making rates

comparable between regions

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MEASUREMENT OF MORBIDITY

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Measurement of morbidity

• Incidence– Occurrence of new cases

• Prevalence– Existence of new and old cases

• Incidence – how many people with the disease are newly diagnosed each year (like video)

• Prevalence - how many people in a population currently have the disease (like snapshot)

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Incidence (நடக்குறது)

X 1000

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Incidence – example

JANUARY 1, 2016 DECEMBER 31, 2016

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Special incidence rates

• Attack rate• Secondary attack rate

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Uses of incidence rate

• For taking action to control disease• More suited for acute or infectious conditions• For research

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Example of use of incidence

This news article is giving only the numerators,Without denominator, it is hard to place much importance

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Example of use of incidence

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Points to remember about incidence

• Refers only to new cases• Not influenced by duration of disease• Refers to a particular time period• Denominator is people at risk

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Prevalence (இருக்குறது)

X 1000

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Types of prevalence

• Point prevalence • Period prevalence

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Point prevalence – example

JANUARY 1, 2016 DECEMBER 31, 2016

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Period prevalence – example

JANUARY 1, 2016 DECEMBER 31, 2016

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Prevalence increases

• Longer duration of disease• Prolongation of life with treatment• Increase in incidence• Immigration of new cases• Better reporting of cases• Emigration of healthy people

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Prevalence decreases

• Shorter duration of diseases• Improved cure rate• Decrease in incidence • Emigration of new cases• Under reporting of cases• Immigration of healthy people

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Uses of prevalence

• Magnitude of disease problems • Identify potential high-risk populations • Administrative and planning purposes, e.g.,

hospital beds, manpower needs, rehabilitation facilities

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Example of use of prevalence

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Example of use of prevalence

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Points to remember about prevalence

• Refers to new and old cases• Influenced by duration of disease• Refers to a particular time period• Denominator is people at risk

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Relation between incidence & prevalence

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Relation between incidence & prevalence

• Prevalence = Incidence X duration

• Incidence = 10 cases/1000 population/year• Mean duration of disease = 5 years• Prevalence = 10 x 5 = 50 per 1000 population

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Summary

• Incidence and prevalence are finer measurements of health as compared to death rates

• They help us to measure the effectiveness of disease control measures

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Take home messages

• Proportion, rate and ratio – basic tools• CDR is a simple measure of death/health• Standardization is needed for comparability• Incidence reflects new cases only• Prevalence reflects new and old cases• Incidence and prevalence are related• Mortality measures are important, morbidity

measures give a better idea of health

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Review 1

• It was reported that the incidence of dengue was increasing every year in Madurai. This could mean all EXCEPT,

a) Control of mosquitoes has failedb) Reporting of dengue cases has improvedc) Treatment for dengue has failedd) Public awareness on dengue has increased

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Review 2

• Prevalence of Diabetes is increasing every year in India. This could mean all EXCEPT,

a) Incidence of DM is increasingb) Reporting of diabetes has increasedc) Diabetic patients are surviving longer due to better

treatmentd) Public awareness on diabetes has increasede) None of the above

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Review 3

• Examples of a disease with high incidence but low prevalence include (multiple options)

a) Acute respiratory infectionb) Acute diarrhoea c) TBd) Leprosy

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Review 4

• Examples of a disease with low incidence but high prevalence include (multiple options)

a) Acute respiratory infectionb) Acute diarrhoea c) TBd) Leprosy

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Review 5

• A new diabetes control programme was introduced in Madurai. After 1 year, the incidence and prevalence of Diabetes increased. This means

a) The programme did not workb) The programme worked c) Data not sufficient

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Review 6

• Disadvantages of crude death rate include (multiple options)

a) Simple measureb) Influenced by the age compositionc) Not comparable between countriesd) All of the above

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Review 7

• Proportional mortality is useful for all EXCEPT

a) Understanding relative importance of diseases as a cause of death

b) Determining measures for reducing preventable mortality

c) Indicating the risk for population from dying due to a particular cause

d) All of the above

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Review 8

• Standardized rates can be calculated for

a) Age b) Sex c) Race d) Literacy ratee) All the above

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Review 9

• Standardized mortality ratio requires all EXCEPT

a) Age specific denom. for interest pop.b) Age specific death rates for standard pop.c) Age specific death rates for interest pop.d) Crude death rate in interest pop.

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Review 10

• A patient with Atherosclerotic heart disease for 7 years developed Acute myocardial infarction last week and died today as a result of myocardial rupture. What is the immediate cause of death?

a) Acute myocardial infarction b) Atherosclerotic heart disease c) Cardiac arrestd) Rupture of myocardium

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Review 11

• Mid year population is not the denominator for (multiple options)

a) Age specific death rateb) Cause specific death ratec) Crude death rated) Proportional mortality ratee) Case fatality rate

THANK YOU

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