2. Session objectives
By the end of the presentation, the student is expected to:
1. Define the morbidity
2. Describe the sources of morbidity statistics
3. Discuss the uses of morbidity statistics
4. Enumerate the measurements of morbidity
5. Describe the measures of Disease Frequency
3. Definition of Morbidity
• Any departure, from a state of physiological or mental
well-being ,whether due to disease, injury or impairment
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4. Sources of morbidity statistics
1. Routine health service record
2. Surveillance records of selected diseases
3. Disease notification
4. Occupational health unit
5. Reports from volunteer workers
6. Reports of Ministry of Health and international organizations
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5. Uses of morbidity statistics
1. Control of infectious diseases
2. Planning for development of preventive services
3. Planning for provision of adequate treatment services
4. Research into etiology and pathogenesis
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6. Measurement of morbidity
Three aspects of morbidity are commonly measured namely :
1. Frequency
2. Duration
3. Severity
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8. Measures of Disease Frequency
Can be expressed as the following format:
1. Ratio
2. Proportion
3. Rate
9. = 5 / 2 = 2.5 / 1
1.Ratio
• A fraction in which the numerator is not part of the denominator.
• A relation in size between two random quantities
• E.g. Sex ratio: Male / Female
10. 2. Proportion
• A fraction in which the numerator is part of the denominator.
• Quantities have to be of same nature
• Proportion always ranges between 0 and 1
12. Proportion
Example:
If You want to calculate the proportion of the COVID 19 cases in X village
The number of adults with COVID19 at a certain time x100
The total number of adults in the village at the same time
• Most fractions in epidemiology are proportions.
13. 3. Rate
• A rate measures the occurrence of some particular event (development of
disease or the occurrence of death) in population during a given time period.
• Describes how rapidly health events are occurring in a population of interest.
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14. 2
----- = 2per 1000 / year
1000
Observed in 1998
- Number EVENTS observed for a given time
- Population in which the events occur (population at risk)
- Includes time
15. Example
• Mortality rate of malaria in ‘X city’ in 1995
• Malaria deaths: 20
• Population in 1995: 100,000
• Mortality rate = 20/100,000 per year
• Rate may be expressed in any power of 10
• 100, 1,000, 10,00, 100,000
16. Measures of Disease Frequency
• Several standard measures are used to measure and describe the frequency of
disease.
• The two main types of measuring the frequency of disease in populations are:
• Incidence
• Prevalence
19. Incidence
• Measures new cases of a disease in a defined population (population-at-risk)
within a specified period of time
• Can be measured as a rate or a proportion.
• Requires a follow-up over time
• Very helpful for etiological/causal inference
20. The population at risk:
• The people who should be able to develop the disease in question
during the time period covered.
Example:
Population at risk in a study of carcinoma of the cervix
21. Two types of incidence
1. Incidence proportion
2. Incidence rate.
22. 1.Incidence proportion
• Also known as:
• Attack rate
• Risk
• Probability of developing disease
• Cumulative incidence
23. Incidence proportion
• Incidence proportion is the proportion of an initially disease-free population
that develops disease, becomes injured, or dies during a specified (usually
limited) period of time
25. Example
• In the study of diabetics, 100 of the 189 diabetic men died during the 13-
year follow-up period. Calculate the risk of death for these men.
• Numerator = 100 deaths among the diabetic men
Denominator = 189 diabetic men
• Risk = (100 ⁄ 189) × 100 = 52.9%
26. • In the outbreak setting, the term attack rate is often used as a synonym for
risk. It is the risk of getting the disease during a specified period, such as the
duration of an outbreak.
• A food-specific attack rate is the number of persons who ate a specified
food and became ill divided by the total number of persons who ate that
food
28. EXAMPLE: Calculating Secondary Attack Rates
• Consider an outbreak of shigellosis in which 18 persons in 18 different
households all became ill. If the population of the community was 1,000,
then the overall attack rate was 18 ⁄ 1,000 × 100% = 1.8%. One incubation
period later, 17 persons in the same households as these “primary” cases
developed shigellosis. If the 18 households included 86 persons, calculate the
secondary attack rate.
• Secondary attack rate = (17 ⁄ (86 − 18)) × 100% = (17 ⁄ 68) × 100% = 25.0%
30. • Incidence rate or person-time rate is a measure of incidence that
incorporates time directly into the denominator. A person-time rate is
generally calculated from a long-term cohort follow-up study, wherein
enrolees are followed over time and the occurrence of new cases of disease
is documented.
32. EXAMPLES
• The diabetes follow-up study included 218 diabetic women and 3,823 nondiabetic women. By
the end of the study, 72 of the diabetic women and 511 of the nondiabetic women had died.
The diabetic women were observed for a total of 1,862 person years; the nondiabetic women
were observed for a total of 36,653 person years. Calculate the incidence rates of death for the
diabetic and non-diabetic women.
• For diabetic women, numerator = 72 and denominator = 1,862
• Person-time rate = 72 ⁄ 1,862
= 0.0386 deaths per person-year
= 38.6 deaths per 1,000 person-years
• For nondiabetic women, numerator = 511 and denominator = 36,653
• Person-time rate = 511 ⁄ 36,653 = 0.0139 deaths per person-year
= 13.9 deaths per 1,000 person-years
34. Prevalence
• Number of all existing cases(new and old cases) of a disease at a particular point in
time or over a period of time divided by the population at risk of having the disease
at that time or midway through the period.
• Very helpful for quantifying disease burden.
• It is a proportion, not a rate.
35. Two types
1. Point prevalence refers to the prevalence measured at a particular point in
time. It is the proportion of persons with a particular disease or attribute on a
particular date.
36. 2. Period prevalence refers to prevalence measured over an interval of time. It
is the proportion of persons with a particular disease or attribute at any time
during the interval.
37.
38. EXAMPLE
• In a survey of 1,150 women who gave birth in X city, a total of 468 reported
taking a multivitamin at least 4 times a week during the month before
becoming pregnant. Calculate the prevalence of frequent multivitamin use in
this group.
• Numerator = 468 multivitamin users
Denominator = 1,150 women
• Prevalence = (468 ⁄ 1,150) × 100 = 0.407 × 100 = 40.7%
40. The relation between prevalence
and incidence
• Prevalence depends on
• Incidence (I)
• Duration of the disease (D)
P = I x D
• Change in prevalence from one time period to another may be the result of
changes in incidence rates, changes in the duration of disease, or both
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41. Incidence and prevalence
• High prevalence and low incidence diseases like;
• Diabetes Mellitus
• Low prevalence and high incidence diseases like;
• Common cold
