mortality rates & standardization

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MORTALITY RATES AND STANDARDIZATION

Presenter : Dr. Vaishnavi CPost graduate, MD community medicineM S Ramaiah medical college, Bangalore

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CONTENTS

Introduction

Frequency measures- Rates, ratios and proportions

Mortality rates

Definition

Parts

Types of mortality rates

Relevance of mortality rates in public health

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CONTENTS

Comparing mortality data

Standardization of mortality rates

Definition

Types

Example

Issues of standardization – advantages and disadvantages

Summary

References

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Introduction

• Statistics are used to summarize the data collected

describe risk

make comparisons

identify high-risk groups

develop hypotheses about the cause(s) of disease

• Most common – frequency measures

Rates, ratios and proportions

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Frequency Measures

• Ratios –

compares the occurrence of a variable in two different groups

Eg:

No. of male children attending a immunization clinic

No. of female children attending a immunization clinic

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Frequency Measures

• Proportions:

A proportion is a ratio which indicates the relation in

magnitude of a part of the whole

Eg:

No. Of male children attending the immunization clinic

Total number of children attending the immunization clinic

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Frequency Measures

• Rates:

Type of frequency measure used with two-category

variables

Measure the occurrence of an event in a population over

time

Reflect groupings of people based on time, place and

person.

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Frequency Measures

• Comprises the following – numerator, denominator, time

specification and multiplier.

• Usually expressed as per 1000 or 1, 00,000 according to

convenience or convention to avoid fractions.

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Frequency Measures

• a ratio to compare two independent groups

• a proportion to compare one group with a larger one to

which it belongs

• a rate to measure an event in a population over time

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Categories of rates:

Crude rates

Specific rates

Standardized rates

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Mortality rates

• Mortality rates measure the frequency of occurrence of death

in a defined population during a specified interval

• Difference in the risk of dying from a disease

• Can serve as measures of disease severity

• Serve as surrogates for incidence rates when the disease being

studied is severe and lethal one.

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Mortality rates

• Essential components of death rates:

• A population group exposed to the risk of death

• A time period

• The number of deaths occurring in the population during that

time period

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Types of mortality rates

Crude Death Rate:

Number of deaths in an area in a year per 1000 population

counted at midyear

Total No. of Deaths from all causes in 1 year X 1000

No. of persons in the population at mid year

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Mortality rates

Specific mortality rates:

• To calculate the rate specifically for a group eg. Age, sex,

ethnic groups

Age specific mortality rate :

No. of deaths among children < 10 years in one year X 1000

No. of children in the population < 10 years of age at mid year

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Mortality rates

• Eg:

Annual mortality rate from leukemia

in children < 10 years of age

= No. of deaths from leukemia in one year in children < 10 years

No. of children in the population < 10 years at mid year

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Mortality rates

Case Fatality Rates:

• What percentage of people who have a certain disease die

within a certain time after their disease was diagnosed

• It is the measure of disease severity

No. of individuals dying during a specified

period of time after disease onset or diagnosis X 100

No. of individuals with the specified disease at that particular

time

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Mortality rates

Proportional Mortality Rate:

• Represents the no. of deaths due to particular cause per 100/

1000 total deaths

• Expressed as percentage

No. of deaths from a specific cause X 100

Total No. of deaths in the population

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Example:

Assume a population of 1,00,000 of whom, 20 are sick with the

disease X and in that year 18 die from the disease X and total

deaths from all causes is 36.

Crude Death rate = 36 per 1 lakh population

Case fatality rate = 18/20 X 100 = 90%

Proportional mortality rate = 18/36 x 100= 50%

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• Single indicator which can signal a broad range of health

problems

• Clearly an index of the severity of a disease from both clinical

and public health stand points

• Also be used as an index of the risk the disease

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Are good reflection of incidence rates under 2 conditions:

• When the case fatality rate is high

• When the duration of disease is short

Eg: Ca. Pancreas is a highly lethal disease and death generally

occurs within months of diagnosis. Hence mortality rates from

ca pancreas can be taken as the incidence of the disease.

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Limitations of mortality data:

• Incomplete reporting of deaths

• Lack of accuracy - inaccuracies in the recording of age and

cause of death

• Lack of uniformity

• Choosing a single cause of death

• Diseases with low fatality

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Comparing mortality in different population

• Important for the evaluation of community health status

• Used to compare two or more populations or one population in

different time periods

• populations may differ in regard to many characteristics & so

comparisons of crude rates can be misleading

• Differences in determinants can distort comparisons between

populations

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Advantages Disadvantages

Crude Death Rate Actual summary rates

Readily calculable

Since populations vary in composition (e.g., age), differences in crude rates difficult to interpret

Specific Rates Homogenous subgroups

Detailed rates useful for

epidemiologic and public health purposes

Cumbersome to compare many subgroups of two or more populations

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Comparing mortality in different population

Two criteria:

• Rates should relate the number of events to

the population at risk

• Because many health outcomes vary by

age, the effect of the population’s age

distribution must be taken into account

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Several techniques for adjusting age-specific rates:

• Direct and indirect standardization

• The comparative mortality index

• Equivalent average death rates

• Life table rates

• Yerushalmy’s index

• Cumulative death rates

• Regression analysis & multivariate techniques

Source: WHO: Age standardization of rates

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Standardization of rates

• Set of techniques used to remove as far as possible the effects

of differences in age or other confounding variables when

comparing two or more variables

• Age and sex are two of the most common variables used for

standardization - called standardized rates

• Involves taking weighted averages of the stratum-specific

outcome measures

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• A standard is a set of weights that is used in taking weighted

average

Eg: if the only stratification variable is age, a standard might be

the amount of person-time or number of persons in a standard

population that fall into each of the age categories

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2 types :

Direct standardization

Indirect standardization

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Direct standardization

• To compare health outcomes among populations that may

have different age distributions

• Used when age specific rates of disease are known for the

populations being compared.

• A standard population is used in order to eliminate the effects

of any differences in age between the two or more populations

being compared

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Standard population

Defined as one for which the numbers in each age and sex

group are known.

"Artificial populations" with fictitious age structures

uniform basis for the calculation of comparable measures for

the respective reference population

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Choice of standard population: considerations

• When several different populations are being compared, a

‘pooled’ standard minimizes the variance of the adjusted rates

• In examining trends, an appropriate standard is one that

reflects the average structure of the population over the time

period

• The standard should be similar to the population of interest

• It should not change frequently (all historic data would need to

be recomputed)

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One of the following is used for a standard age distribution:

• The distribution of one of the populations being compared

• An independent standard, e.g. national population in an

arbitrarily chosen year

• A distribution constructed by combining the populations, e.g.

by averaging or totalling

Steps in calculating age adjusted rate:

Step 1:

A hypothetical “standard population” is created

Step 2:

The age-specific rates of the population whose crude death rate

has to be standardized is applied to the standard population

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Steps in calculating age adjusted rate:

Step 3: For each age group an expected number of deaths in the

standard population is obtained.

Step 4: The expected number of deaths are added together for all

age groups to get the total expected deaths.

Step 5: Divide the total expected deaths by the total of the

standard population, which yields the standardized or age-

adjusted rate.

Age groups

Population A

No. Of deaths in each age group

Age specific death rate

Age groups

Population B

No. Of deaths in each age group

Age specific death rate

B1 A1 B2 A2

CDR = A1/B1 CDR =A2/B2

Age groups

Std population

Age specific death rates

Expected deaths

Age groups

Std population

Age specific death rates

Expected deaths

C1 D1 C1 D2

ADR1 = D1/C1 ADR2 = D2/C1

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Direct Standardization

• Standardised rate :

No. of estimated events for a community X 100

Total population of the community

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Example 1:

Population A Population B

No. of deaths 862 1130

Total population 900000 800000

Crude mortality rate

(per 1,00,000

population)

96 142

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Example 1:

Population A Population B

No. of deaths 862 1130

Total population 900000 800000

Crude mortality rate

(per 1,00,000

population)

96 142

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Age group Population A Population B

Total

number

No. of

deaths

Death

rates/1

00000

Total

number

No. of

deaths

Death

rates/10

0000

30-49 500000 60 12 400000 30 8

50-69 300000 396 132 200000 400 200

>70 100000 406 406 200000 700 350

Total 900000 862 96 800000 1130 142

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Standard population

Age Population

30-49 900000

50-69 500000

>70 300000

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Age group Standard population

Age specific mortality rates – Pop- A

Expected number of deaths I

Age specific mortality rates- pop- B

Expected number of deaths II

30-49 900000 12 108 8 72

50-69 500000 132 660 200 1000

>70 300000 406 1218 350 1050

Total 1700000 96 1986 142 2122

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Age group Standard population

Age specific mortality rates – Pop- A

Expected number of deaths I

Age specific mortality rates- pop- B

Expected number of deaths II

30-49 900000 12 108 8 72

50-69 500000 132 660 200 1000

>70 300000 406 1218 350 1050

Total 1700000 96 1986 142 2122

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• Age adjusted rates:

No. of expected deaths X 100000

Total standard population

• Population A = 1986 /1700000= 117 per 100000 population

• Population B = 2122/1700000= 125 per 100000 population

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Example 2:

Age Total subjects Cases Controls

No Heavy

smokers

% No Heavy

smokers

%

40-49 500 400 200 50 100 50 50

50-59 500 100 10 10 400 40 10

Total 1000 500 210 42 500 90 18

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Age adjustments are carried out

• By combining number of subjects in both the age groups

(500+500 = 1000) to create a standard population

• Applying the observed age-specific proportions of heavy

smokers to the same standard population

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Age Subjects Expected number of heavy smokers

Cases controls

40-49 500 500X50/ 100 = 250 500X50/100 = 250

50-59 500 500X 10/100 = 50 500X 10/100 = 50

Total 1000 300 300

Standardized rates

Cases= 300X 100/1000 = 30

Controls = 300X 100/1000 =30

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Direct Standardization

• Direct method requires the knowledge of age specific rates of

the events under comparison for each of the communities as

well as the standard population age structure.

• Sometimes, the above data may not be available or the

population in different age or sex groups or may be too small

resulting in large fluctuating of the age specific rates through

the presence of only a few events

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Indirect method of standardization

• Requires the knowledge of the age and sex structure of the

population as well as the age and sex specific rates of events

under consideration for the standard population

• Used when the number of deaths for each age specific stratum

is not available

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• Steps:

• Step 1: Choose a reference or standard population.

• Step 2: Calculate the observed number of deaths in the

population (s) of interest

• Step 3: Apply the age-specific mortality rates from the chosen

reference population to the population(s) of interest.

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• Step 4: Multiply the number of people in each age group of the

population(s) of interest by the age-specific mortality rate in

the comparable age group of the reference population.

• Step 5: Sum the total number of expected deaths for each

population of interest.

• Step 6: Divide the total number of observed deaths of the

population(s) of interest by the expected deaths

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Standardized Mortality Ratio (SMR):

• The simplest and most useful form of indirect standardization

• Ratio of the total number of deaths that occur in the study

group to the number of deaths that would have been expected

to occur if that study group had experienced the death rates of

the standard population

SMR = Observed Deaths X 100

Expected Deaths

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Age groups Population A Observed No of deaths in each age group (obs1)

Population B Observed No of deaths in each age group(obs2)

A1 B1

CDR1 = Obs 1/A1

CDR2 = Obs 2/B1

Age groups Std Age specific death ratesS

Population A

Expected deaths in each group

Population B Expected deaths in each group

E1 = S* A1 E2 = S*B1

SMR 1 = Obs 1/ E1 SMR 2 = Obs 2/ E2

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Example 1

• In a population of 5,34,533 miners in a country, 436 deaths

from tuberculosis occurred in 2000.

• The question of interest is whether this mortality experience

from TB is greater than, less than or about the same as that

expected from the general population

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Age (yr) Estimated population of miners

Observed deaths from TB in miners

Death rate/1,00,000 for TB in general population

20-24 74598 10 12.26

25-29 85077 20 16.12

30-34 80845 22 21.54

35-44 148870 98 33.96

45-54 102649 174 56.82

55-59 42494 112 75.23

Total 534533 436

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Observed deaths from TB in miners


20-24 74598 10 12.26

25-29 85077 20 16.12

30-34 80845 22 21.54

35-44 148870 98 33.96

45-54 102649 174 56.82

55-59 42494 112 75.23

Total 534533 436

Std population

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Expected deaths from TB in miners

20-24 74598 12.26 9.14

25-29 85077 16.12 13.71

30-34 80845 21.54 17.41

35-44 148870 33.96 50.55

45-54 102649 56.82 58.32

55-59 42494 75.23 31.96

Total 534533 181.09

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SMR = 436 / 181.09 X 100

= 241

• SMR = 100 - observed death is same as expected death.

• SMR < 100 - observed death is less than expected death.

• SMR > 100 - observed death is more than expected death.

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Example: 2 Age Population Std population –

age & sex specific death rates

Estimated deaths

Males Females Males Females Males Females0-4 734 685 58.3 70.2 42.8 48.1

5-14 1494 1311 4.5 5.3 6.7 6.9

15-19 471 352 2.1 4.2 1.0 1.5

20-24 397 399 3.9 5.5 1.5 2.2

25-29 386 362 3.7 5.5 1.4 2.0

30-34 339 327 4.1 6.4 1.4 2.1

35-39 293 239 6.5 6.1 1.9 1.5

Total 4114 3675 56.7 64.3

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• Index death rate for the population =

total estimated events X 1000

total population

Index rate = 121/ 7789 X 1000

= 15.5

Standardising factor = Overall death rate of standard population /

Index rate

= 23.91 / 15.5

= 1.54

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• Standardised death rate for the community;

Observed rate X standardizing factor

= 12.7 X 1.54

= 19.6

Crude – 12.7

Standardised rate = 19.6

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Advantages of standardization:

• Summarizes stratum specific rates

• Unconfounded comparison of population

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Issues in the use of standardisation:

• Standardised rates are used for the comparison of two or more

populations; they represent a weighted average of the age

specific rates taken from a 'standard population' and are not

actual rates.

• The direct method of standardisation requires that the age-

specific rates for all populations being studied are available

and that a standard population is defined.

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Issues in the use of standardisation:

• The indirect method of standardisation requires the total

number of cases

• As the choice of a standard population will affect the

comparison between populations, it should always be stated

clearly which standard population has been applied.

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Summary

• Most common statistical measures – Rates, ratios, proportions

• Mortality rates – relevance in public health

• Crude death rates are easy to calculate but distort the

comparisons between population

• Hence standardization is used to enable comparisons

• 2 types – direct and indirect

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Summary

Direct Indirect

Target Population Group specific rates Group population

sizes

Standard

Population

Group population

sizes

Group specific rates

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Summary

Advantages Disadvantages

Summarizes stratum specific rates

Fictitious values

Unconfounded comparison of population

Values depend on the choice of standard population

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References

• Gordis L. Epidemiology. 4thed. Philadelphia (USA): Elsevier

Saunders; 2009. p.

• RothmanKJ, Greenlands S, lash TL. Modern Epidemiology. 3rd ed.

Philadelphia (USA): Lippincot Williams and Wilkins. 2008. P.

• Park K. Textbook of Preventive and Social Medicine. 22nd ed.

Jabalpur (India): Banarasidas Bhanot; 2013.p.

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References

• Detels R, Beaglehole R, Lansang MA, Gulliford M. Oxford

Textbook of Public Health. 2nd ed. United Kingdom: Oxford

University press;2001. p.

• Hennekens CH, Buring JE. Epidemiology in medicine. 1st ed.

Boston: Little, Brown and Company; 1987.

• Rao NSN, Murthy NS. Applied Statistics in Health sciences. 2nd ed.

New Delhi (India): Jaypee Brothers Medical Publishers; 2010.

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References

• Age standardization of rates: A new WHO standard. World Health

Organisation [online] 2001 [cited on 2016 May 1];Available from:

URL: http://www.who.int/healthinfo/paper31.pdf

• The use of epidemiological tools in conflict affected populations:

open access educational resources for policymakers. World Health

Organisation [online] 2002 [cited on 2016 Apr 24]; Available from:

URL:http://www.who.int/hac/techguidance/tools/disrupted_sectors

/adhsm_mod14_en.pdf

http://www.who.int/healthinfo/paper31.pdf

http://www.who.int/hac/techguidance/tools/disrupted_sectors/adhsm_mod14_en.pdf









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References• Standardization of rates and ratios. Concepts and basic methods for

deriving measures that are comparable across populations that differ

in age and other demographic variables [Online] [ cited on 2016 Apr

20];Available from: URL:

http://www.epidemiolog.net/evolving/Standardization.pdf

• West MD, Kanchanaraksa S. The Direct and Indirect Methods of

Adjustment. John Hopkins School of Public Health [online] [cited

on 2016 Apr 23]; Available from: URL:

http://ocw.jhsph.edu/courses/fundepi/PDFs/Lecture7.pdf

http://www.epidemiolog.net/evolving/Standardization.pdf

http://ocw.jhsph.edu/courses/fundepi/PDFs/Lecture7.pdf

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References

• Standardization of rates. Boston University School of Public Health

[online] 2006 [Cited 2016 Apr 21]; Available from: URL:

http://sphweb.bumc.bu.edu/otlt/MPH-Modules/PH/Woburn/Woburn8.html

• Curtin LR, Klein RJ. Direct Standardization (Age-Adjusted Death

Rates). Healthy People 2000. CDC [Online] 1995 [Cited on 2016

Apr 24]; Available from: URL: www.cdc

.gov/nchs/data/statnt/statnt06rv.pdf



http://www.cdc.gov/nchs/data/statnt/statnt06rv.pdf



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THANK YOU

mortality rates & standardization

Health & Medicine