measures of disease frequency1

7/28/2019 Measures of Disease Frequency1

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MEASURES OF DISEASE

FREQUENCY

BL


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Introduction

Epidemiology deals with population

Deals with counts

Characterize disease occurrence by person, timeand place

The number of persons with disease is also

affected by length of time during which thepopulation was observed for the occurrence ofdisease, sometimes called the risk period


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Describing disease occurrence

Counts

Ratios

Proportions

Rates


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Count

Basic measure of disease occurrence

Absolute numbers

Basic requirement in calculating rates andratios

Counts

Incident cases

Prevalent cases


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Incident case - refer to the number of newcases during an interval of time

Prevalent case - refer to the number ofexisting cases at one point in time


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Limitation of count

Difficult to measure the magnitude of disease

as compared to the population Difficult to compare to other population

Comparison can be done by ratio


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Ratio

a:b

a/(b x k)


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Rate

Numerator is part of denominator

a / (a+b)


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Rate

Infant Mortality Rate Number of infantdeaths per 1000 live births.

Number of infant deaths in acalendar year

Number of livebirths in acalendar year

---------------------------------------- x 1000IMR =


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Measures of morbidity

Prevalence


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Incidence


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Incidence


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Measures of Mortality

Mortality measures are those that use thenumber of deaths in the numerator


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Crude Death Rate

The crude death measures the impact ofmortality in a population

It tells us how fast deaths are occurring ina specific population.


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Cause-Specific Death Rate

If the total deaths in the numerator of thecrude death rate is replaced by the

number of deaths from a specific cause,we obtain a cause-specific death

This measures how fast individuals in a

population are dying from a specific cause


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Cause-Specific Death Rate

The cause-specific death rate measuresthe impact of a certain cause in the

population in terms of mortality.


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Death Rate for Specific Subgroups

The numerator are the number of deaths inthe specific subgroup while thedenominator is the correspondingpopulation size of this group

For example, the death rate for males willhave for the numerator male deaths andfor the denominator population size of the

males Specific rates are the more appropriate

measures to use for comparing populationsthat have different distributions


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Specific Death Rate


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Case Fatality Rate

The numerator includes those who die ofa certain disease while the denominator

includes the number of cases of thisdisease

The case-fatality measures the probability

of an individual dying form a specificdisease once it sets in.


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Case Fatality Rate


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Proportionate Mortality Ratio

The proportionate mortality ratio for aspecific cause is the proportion of all

deaths that is attributed to this cause This measures the relative contribution of

a specific cause to the deaths in a

population


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The proportionate mortality ratio is usedfor comparing the importance of causes of

death within a population. The use of this measure for comparing

different populations can be misleading


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Note: Swaroops Index


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Crude, Specific and Adjusted Rates

Crude Rates

single (summary) measures that are

computed for the population These are obtained by simply dividing the

number cases (diseased individuals or deaths)by the size of the population from wherethese cases come from in a specified period oftime


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Specific Rates

are derived for subgroups of the population

These are computed in the same manner ascrude rates except that the items counted inthe numerator and the denominator arelimited to those who meet the criteria formembership in the subgroup


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Adjusted Rates single summary measures that have been

adjusted for the purpose of removing theeffects of differences in the populations (e.g.age differences) when comparing thesepopulations.

One form of adjustment is to use a singlestandard population and compute for theover-all summary rate in this standardpopulation using the specific rates from a

population under study

Crude Death Rate and Age Specific Death Rate (Manila)


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Crude Death Rate and Age-Specific Death Rate (Manila)

Age No. of death Population Death Rate

0-29

30-59

60+

Total

Crude Death Rate and Age-Specific Death Rate (Quezon)

Age No. of death Population Death Rate

0-2930-59

60+

Total

4,200

6,800

7,650

18,700

850,000

680,000

170,000

1,700,000

0.005

0.010

0.045

0.011

5,5008,000

4,500

18,000

1,100,000800,000

100,000

2,000,000 0.009

0.0050.010

0.045

Expected number of deaths by age in Manila using the standard


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Expected number of deaths by age in Manila using the standardpopulation

Age Death Rate Population No. of death

0 29 1,950,00030 59 1,480,000

60 + 270,000

Total 3,700,000Expected number of deaths by age in Quezon City using thestandard population


0 29 1,950,000

30 59 1,480,000

60 + 270,000

Total 3,700,000

0.0050.010

0.045

9,75014,800

12,150

36,700

0.005

0.010

0.045

9,750

14,800

12,150

36,700


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To get the age-adjusted death rate, thetotal expected deaths is divided by the

total population of the standard. Here we have 0.0099

(=36,700/3,7000,000) for both Manila and

Quezon City. Now we see that theadjusted-rates are equal. We haveeliminated the effects of age-differences.


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Indirect standardization

one also chooses a standard population.

if age is the variable for adjustment,

instead of using the age distribution ofthis standard population, we use age-specific rates in this standard population.


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Indirect standardization

These rates are then multiplied to the age-specific population under study to arrive at the

expected number of cases in this population hadit had the same rates as that of the standardpopulation

The total number of cases observed in the

population under study is divided by the derivedexpected number of cases to obtain thestandardized mortality ratio


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Age Distribution and Crude Death Rate for Makati

Age Population

0 29 480,000

30 59 280,000

60 + 40,000

Total 800,000

Total deaths = 8,000

Crude Death Rate = 0.010


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We would like to compare the death ratein Manila and Makati.

If we take the age specific death rates inManila as in the above example anddetermine what would be the expected

number of cases in Manila with apopulation similar to Makati


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Expected number of deaths by age in Manila using Makati

population


0 29 480,000

30 59 280,00060 + 40,000

Total 800,000

0.005

0.0100.045

2,400

2,8001,800

7,000


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Standardized Mortality Ratio


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We conclude that the observed number ofcases in Makati exceeded that of what

would be expected if the age-specific ratesin Makati had been similar to Manila.


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Thank You

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