1. SECTION IV
MEASURES OF DISEASE
FREQUENCY
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2. Rate
• Rate is the basic measure of disease occurrence
because it is the measure that most clearly
expresses probability or risk of disease in a
defined population over a specific period of time
• A rate measures the occurrence of some
particular event in a population during a given
time period
• Categories:
• Crude Rates
• Specific Rates
• Standardized Rates
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3. Rate….
• In a rate Numerator is part of Denominator
• Rate has 5 components
– Numerator (Number of people, episodes…)
– Denominator (Population at risk, Mid-year
population, Person-Time or total events…)
– Time specification.
– Place specification (at a given place..)
– Constant or multiplier…
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4. Crude Rates
• Actual observed rates
– Birth rates
– Death rates
• Also known as unstandardised sates
• Portrays information in a single figure
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5. Crude Death Rate
• Simplest measure of mortality
• Summarizes the effects of two factors
– Population composition
– Age specific death rates
Number of deaths in a specified period
CDR= X10n
Average total population during that
period (mid year population)
• Limitation
– No comparability
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6. Specific Rates
• Actual observed rates due to
– specific causes
e.g tuberculosis
– Specific groups
• Age
• Gender
– During Specific time periods
• Annual
• Monthly
• Weekly
• Specific Death rates
– Most useful single measure of mortality
– Helps us in identifying risk groups
– Permit comparisons between different causes within the same
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7. Standardized Rates
• Obtained by direct or indirect methods of
standardization eg age and sex standardised
rates
• Standard population
– The population for which the numbers in each age
and sex are known
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8. Direct standardisation
• Combining the two populations in both the
age groups to create a standard population
• Then applying the observed age specific rates
to the standard population.
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9. Indirect Standardisation
• Standardised mortality ratio (SMR)
– Occupational groups
• Compares the mortality in a study group eg occupational
group with the mortality that the occupational group would
have had if they had experienced the national mortality rates
• If ratio > 100
– Occupation would appear to carry a greater mortality risk than that of
the whole population
• If ratio < 100
– Occupation risks is less than the for the whole population
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10. Ratio
• It expresses a relation in size between two
random quantities
• Numerator is not a part of the denominator
• Ratio is the result of dividing one quantity by
another
– X : Y or x/ y
• Vital index
• Doctor population ratio
• Child woman ratio
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11. Proportion
• Is a ratio which indicates the relation in
magnitude of a part to the whole.
• The numerator is always a part of the
denominator
• Usually expressed as ……
Males
%
Female + Male
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12. Proportion…..
• Proportional Mortality From Specific Causes
Number of deaths from the specific disease in a year
100
Total deaths from all causes in that year
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13. Numerator & Denominator
• Numerator
– Refers to the number of times an event eg
sickness, birth, death, episodes of sickness has
occurred in a population, during a specific time
period
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14. •
Numerator & Denominator….
Denominator
– Related to the population
• Mid year population…….
• Population at risk
– Is restricted solely to those who are capable of having or acquiring the disease
or condition in question
– General Fertility Rate (15 – 49 years of age )
• Person Time
– Person Years
– Person weeks
• Person Distance
– Person Miles
• Subgroups of the population
– Age, occupation
– Related to total events
• IMR
• Case fatality ratio
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15. Measures of Morbidity
• Incidence
• Prevalence
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16. Incidence
• It is defined as the number of NEW cases occurring
in a defined population during a specific period of
time.
Number of new cases of a specific disease in a given time period
• I R= X 1000
Population at risk during that period
• Attack Rate (At risk for only a limited time)
• Secondary attack Rate (Incidence after the primary
case)
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17. Incidence Rate ….
(Incidence calculated for populations that are at risk for only a limited time.
Weekly, monthly or further duration of epidemic lasting less than one year.
• Attack Rate
Number of new cases of a specified disease during a specified time interval
Total population at risk during the same interval
X 100
3. Secondary Attack Rate
Number of new cases after the primary cases during a specified time interval
Total population (minus primary cases) at risk during the same interval
X 100
The attack rate reflects the extent of the epidemic &
Secondary Attack rate reflects the infectivity of the cases.
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18. Uses of Incidence Rate
1. It is helpful for research into aetiology,
pathogenesis and distribution of the diseases.
2. It is helpful to control the diseases.
3. It is helpful in evaluating the efficacy of
preventive and therapeutic measures.
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19. Prevalence Rate
It is calculated as
No of people with the disease (old & new) or condition
n
=
at a specified time
X 10
No of people in the population at risk at that specified time
Data on the population at risk are not always
available and in many studies total population in the
study area is used as an approximation.
Examples:
1. Point Prevalence
2. Period prevalence
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20. TYPES OF PREVALENCE RATES
• Point prevalence
Number of all current cases (old and new) of a specified disease
X 100
existing at a given point in time
Estimated population at the same point in time
• Period Prevalence
Number of existing cases (old and new) of a specified disease
during a given period of time interval
X 100
Estimated mid-interval population at risk
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21. Relationship between prevalence and
incidence
• P=IxD
= incidence x mean duration
• Example (for a stable condition)
• Incidence = 10 cases per 1000 population per year
• Mean duration of disease = 5 years
• Prevalence = 10 x 5 = 50 per 1000 population
• Conversely, it is possible to derive incidence and
duration
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22. Prevalence Rate
Prevalence is increased by following factors
Longer Duration of illness
Prolongation of life of the patient without cure
Increase in new cases
In-migration of cases
In-migration of susceptible people
Out-migration of healthy people
Improved diagnostic facilities
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23. Prevalence Rate (Contd)
Prevalence will be decreased by the
following factors.
– Shorter duration of disease
– High case-fatality rate from the disease
– Decrease in new cases (decrease in the incidence)
– In-migration of healthy people
– Out migration of cases
– Improved cure rate of cases.
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24. USES OF PREVALENCE
1. Prevalence help to estimate the magnitude of
health or disease problems in the community and
identified the population at risk. (Burden of
Disease)
2. Prevalence rates are helpful for administrative and
planning purposes for Example:
a. Human Resource
b. Hospital Beds
c. Rehabilitation facility
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25. EXERCISE
Q#1. Of the 75 persons who attended the picnic, 46
subsequently developed gastroenteritis.
a. What is the Attack Rate?
b. Is this primary or secondary attack rate?
Q#2. 7 cases of hepatitis A occurred among 70 children
attending a child day care centre. Each infected child
came from a different family. The total number of
persons in the 7 affected families was 32. one
incubation period later, 5 family members of the 7
infected children also developed hepatitis
a. Calculate the secondary attack rate.
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26. Answers
1.a Attack Rate = (46/75) X 100 =61.33%
1.b It is a primary attack rate
2. Secondary attack rate = (5/25) X 100= 20%
At risk population = 32 - 7 = 25
Cases= 12- 7 = 5
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27. Q#1. In a bulk of hundred children out of whom 28
are immunized 2 of them get measles
simultaneously. Subsequently 14 get measles.
Assume the efficacy of the vaccine to be 100%.
What is the secondary attack rate?
1. 5%
2. 10%
3. 20%
4. 21.5%
5. 19.4%
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28. Answer is
Total children=100
No. of immunized=28
No. of un-immunized=100-28=72
No. of primary cases=2
Secondary attack rate=14/(72-2)=14/70 X100 =20%
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29. Q#2. A village has total of 100 under-five children. The
coverage with measles vaccine in this age group is 60%.
Following the occurrence of a measles case in a child
after a visit outside, twenty- six children developed
measles. The secondary attack rate of measles is:
1. 25%
2. 40%
3. 50%
4. 65%
5. 66%
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30. Answer is
Total No. of children=100
No. of immunized =60
No. of un-immunized=40
Primary case=1
Secondary attack rate=26/(40-1)=26/39 X100= 66%
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31. Q#3. In a population of 1000, measles coverage is
60%, one child goes out of station and comes
back with measles from whom 20 more
children get measles. Secondary attack rate of
measles is:
1. 0.65%
2. 5%
3. 6%
4. 6.5%
5. 7%
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32. Answer is
Total No. of children=1000
No. of immunized =600
No. of un-immunized=400
Primary case=1
Secondary attack rate=20/(400-1)=20/399
X100= 5%
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33. 1. BLACK LINE INDICATES CASES
2. BROKEN LINE INDICATES MIGRATED PEOPLE
3. CONTINUOUS LINE INDICATES HEALTHY PEOPLE
4. + SIGN INDICATES DEATH
Q#1 Calculate the average duration of the disease.
Q#2 16/01/2012 the prevalence rate Fawad - Biostats - Rates, ratios &
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34. Answers
1. The average duration of disease is the total number of years of
disease divided by the number of cases, i.e. (10/3) = (3.3) years.
2. a. The prevalence rate depends on the point in time at which the
study takes place; at the start of year (4), for example, it is the
ratio of the number of people with the disease (2) to the number
of people in the population observed at that time (6), i.e. (33)
cases per (100)persons.
b. P=I X D
incidence rate=9.1 cases/100 person years
Duration = 3.3 years
Prevalence = 9.1X 3.3 = 30 cases/ 100 population
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35. Exercise
Q#1. In a population of 5000 incidence of disease is 100
in 1 year duration of disease studied for 2 years
calculate prevalence:
1. 10/1000
2. 40/1000
3. 80/1000
4. 400/1000
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36. Answer is
2. 40/1000
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37. Q#2. In a stable situation:
1. Incidence = Prevalence + Duration
2. Prevalence = Incidence X Duration
3. Incidence = Prevalence X Duration
4. Prevalence = Incidence + Duration
5. Prevalence = Duration - Incidence
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38. Answer is
2. Prevalence = Incidence X Duration
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39. Q#3. Which of the following measures is used
frequently as a denominator to calculate the
incidence rate of disease?
1. Number of cases observed
2. Number of new cases observed
3. Number of asymptomatic cases
4. Person-years of observation
5. Persons lost to follow-up
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40. Answer is
4. Person-years of observation
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41. Questions# 4-5
The results of a study of the incidence of pulmonary
tuberculosis in a village in India are given in the table
below. All persons in the village are examined during
two surveys made 2 years apart, and the number of
new cases was used to determine the incidence rate.
Category of Household at Number of Number of new
First survey Persons cases
With culture positive case 500 10
Without culture positive 10,000 10
case
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42. Q#4 what is the incidence of new cases per 1000
persons years in households that had a culture
positive case during the first survey?
1. 0.02
2. 0.01
3. 1.0
4. 10
5. 20
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43. Answer is
10 new cases =10 cases/1000 persons years
500 persons x 2 years
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44. Q#5 what is the incidence of new cases per 1000
persons years in households that did not have a
culture positive case during the first survey?
1. 0.001
2. 0.1
3. 0.5
4. 1.0
5. 5.0
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45. Answer is
10 new cases =0.5 cases/1000 persons years
10,000 persons x 2 years
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46. Fawad - Biostats - Rates, ratios &
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