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Fundamentals of Epidemiology
Measures of morbidity and mortality used in
Epidemiology
Afolabi Olusegun MBChB (Ife), MPH (Hadassah), FMCPH
Levels of measurement
Nominal
Ordinal
Interval
Ratio
Absolute and Relative measures
Absolute: age, weight, height
Relative: BMI, Ratio of total cholesterol
usually relative to size and
characteristics of a population
Different measures
Ratios express a quantity relative to another
Proportion : Numerator is part of the
denominator
Rate : classically is a ratio of a change in a
quantity to a change in another quantity
with denominator often being time
Ratio
Mortality: Death to case ratio
Morbidity: Risk ratio; Rate ratio; Odd ratio;
Period prevalence
Proportion
Mortality: proportionate mortality
Morbidity: Attack rate, Point prevalence,
Attributable proportion
Rate
Mortality : Crude mortality rate, case fatality
rate, age specific mortality rate, infant
mortality rate
Morbidity: person time incidence rate
RATE: Measure of the frequency with which
an event occurs in a defined population during
a given length of time
(expressed in terms of some arbitrary constant
- %, /1000, 100,000, etc)
Rates may also relate special events to total events
eg. CFR (not time dimensioned)
Comparison of Rates
Advantages Disadvantages
Actual summary Differences in pop.
Crude rates; compositions make
Rates Readily calculable interpretation difficult
Addresses Cumbersome if there
Specific homogeneous are many subgroups;
Rates subgroups; No summary figure
Useful for PH
Summary Fictional rates;
Adjusted measure; Magnitude depends on
Rates Permits unbiased standard pop;
comparisons Hides subgroup diff’s
Measures of health status
** Measures of frequency:
** Measures of association
** Measures of potential population impact
How do we measure dichotomies?--diseases,
disorders, death …
Measures of frequency: eg morbidity—
prevalence and incidence
Prevalence rate is defined as:
The proportion of a population-at-risk
affected by a “disease” at a specific
point in time
Prevalence rate (P) is calculated by:
# of people with the disease
at a specific time x10n
# of people in the population
at risk at the specified time
Types of Prevalence Rates
• Point Prevalence rate =
# persons with the condition at a point
total number of persons in time
• Period Prevalence rate =
# persons with the condition in a specified
total number of persons period of time
Point Prevalence
• Calendar time (ex. Nov 12, 2001)
• Babies born between Jan 1 – Dec 30, 2001
– Of 20,000 born, 60 had symptoms of malformation
• Women who enter a study when they show up
at the clinic and are examined only once
Prevalence question
Interview question: Type of measure:
Do you currently have • Point prevalence?
asthma? • Period prevalence?
• Lifetime prevalence?
Prevalence question
Interview question: Type of measure:
Have you had asthma • Point prevalence?
in the last 10 years? • Period prevalence?
• Lifetime prevalence?
Prevalence question
Interview question: Type of measure:
Have you ever had • Point prevalence?
asthma? • Period prevalence?
• Lifetime prevalence?
Factors influencing observed prevalence rate
• Longer duration of the disease
• Prolongation of life without cure
• Increase in new cases (incidence / risk)
• Out-migration of non-diseased people
• In-migration of susceptible/diseased people
• Improved diagnostic &/or reporting
Factors influencing observed prevalence rate
• Shorter duration of the disease
• Increased case-fatality rate
• Decrease in new cases (incidence / risk)
• In-migration of “healthy” people
• Out-migration of diseased people
• Improved cure rate
Uses
• Pretest probability
• Community diagnosis
• Basis for decision making and planning
• Provide clues for etiology
Question
• A health centre needs information for use in planning a
home care program for people who are too disabled to leave
their houses: for example, how many cases can be expected
to be under care at a given time, and what is the total
number of cases that will be treated during a year? The
following information is obtained from the agency that has a
program in a similar neighborhood. At the beginning of
1999 the population size was 24,000, and at the end of the
year it was 26,000. At the beginning of 1999 there were 96
house bound patients, 20 of these died during 1999, and 4
were moved elsewhere. Another 40 people became house
bound during 1999, and 8 of them died during the year.
Calculate the point prevalence rates at the beginning and end of 1999 and
the period prevalence rate in 1999?
Solution
• Point prevalence rate was 96/24,000 *1000
At the beginning of 1999
• At the end of the year was [(96+40-20-4-
8)/26000]*1000
• To calculate Period prevalence, we either use the
mid year population if given or the average of the
population at the beginning and the end.
(24,000+26,000)/2 =25,000
Period prevalence = [(96+40)/25,000]*1000
Incidence
1. Counts
2. Risk [Cumulative incidence]
3. Rate [Incidence density, person-time incidence]
Cumulative incidence:
Is a proportion—
N of new cases (numerator) in a fixed
group at risk (denominator) over
defined time period i.e. risk
- attack rate (eg food poisoning)
- event rate—new + recurrent cases
- lifetime incidence, eg incidence to age
75
Incidence density or person-time
incidence:
a ratio, a true rate (not a proportion)
N of new cases in a changing population
observed for varying length of time
expressed as N of cases / person-time
Eg NSAID-Alzh NEJM 2001;345:1022-9, HRT-CHD
Ann Intern Med 2001;135:1-8
Rate analagous to speed of car (rate of
change, eg 100 km/hr)
--Instantaneous
--Potential
--Average
Risk analagous to distance covered
--1/2 of the distance
--1/10 of the distance
Incidence (cont’)
How do we calculate incidence?
*Cumulative incidence (CI)
CI = I / N0
CI = I / [N0 -(w/2)] ie survival analysis
CI ~ 1 - e (-PTI* t ) ie calculated from rate
(based on relationship between rate and
risk—see ‘sea’ of person-time)
Incidence (cont’)
Incidence density (ID), person-time incidence
(PTI)
PTI = I / ti ti = PT
PTI = I / N (t) ‘ordinary’ incidence
PTI = ID = average incidence density or
average rate for a given period
Incidence (cont’)
ID (PTI) is a ratio not a proportion
CI is a proportion
Infant mortality example:
Is infant mortality a cumulative incidence
or person-time incidence measure?
Incidence (cont’)
Is infant mortality a cumulative incidence or
person-time incidence measure?
IM as usually calculated is a PTI rate:
N of deaths within 12 months of birth divided by the N of
live births in a calendar year—say 2003. Why is this a rate
and not a proportion? Hint--What have we estimated in the
denominator? PT
How would you determine infant mortality as
a CI measure?
Incidence (cont’)
How would you determine infant mortality as
a CI measure?
IM obtained by following a cohort of babies
from birth to 1 year is a CI measure --a
proportion ie the numerator is part of the denominator
Calculations of incidence from example
So we can estimate risk in 3 ways:
1. Fixed cohort, no attrition CI=I/N0
2. Actuarial method CI= I/(N0-w)
2
3. Incidence density
CI 1 - e -ID * t = PTI·t / [(PTI · t/2)+1]
CI ~ ID · t when this product is small
~ < 0.10-- 0.20
CI ~ ID in py when t (duration) is 1 yr
Uses of incidence
- Prediction individuals
- Clinical dx
- Etiologic studies (cause)
- Community dx population
- Prevention/evaluation
outcome evaluation of a program
ie change in incidence of new or
recurrent events
Relationship between incidence
and prevalence
Gordis
Relationship between incidence
and prevalence
Relationship between incidence
and prevalence
Prevalence - Incidence
Screened Prevalence Annual Duration
population (per 1000) Incidence (years)
(per 1000)
Onetown 100
Twotown 60
Prevalence - Incidence
Screened Prevalence Annual Duration
population (per 1000) Incidence (years)
(per 1000)
Onetown 100 4
Twotown 60 20
Prevalence - Incidence
Screened Prevalence Annual Duration
population (per 1000) Incidence (years)
(per 1000)
Onetown 100 4 25
Twotown 60 20 3
Prevalence - Incidence
Screened Prevalence Annual Duration
population (per 1000) Incidence (years)
(per 1000)
Onetown 100 4 25
Twotown 60 20 3
Prevalence Incidence x Duration
Mortality studies
Basic and commonly available information
for characterizing a population and
community dx
Sources: *death notifications (DC)
[numerator]
*counts, census in defined areas
[denominator]
Mortality rates
1. Proportional mortality
denominator - all death
2. Case fatality
denominator - all cases of disease
i. tells you how important - relatively
ii. tells you how dangerous
3. Crude mortality—if ‘ordinary incidence’
then the denominator is the average
population, typically during a year
Mortality rates (cont’)
4. Stratum specific mortality
eg age, sex specific - infant mortality
5. Disease specific mortality
denominator - same as 3 (above)
6. Prevalence at death
denominator - total number of
autopsies
Proportional mortality--examples
Mortality* from cancer in black mine workers
(S. Africa) and US blacks
US Black Bantu Site
3.0 12.7 liver
61.5 1.3 other
64.5 14.0 all
* rates per 100,000 PY, not age adjusted
Proportional mortality (cont’)
Proportional mortality ratio = 19
(91% of cancer in Bantu due to PHC vs 4.7% in
US blacks)
Rate ratio = 12.7 / 3 = 4.2
• Case Fatality rate (percent)=
No. of individuals dying during a specified period of time after disease onset or diagnosis *100
No. of individuals with a specified disease
• Crude Death Rate=
Total no. of deaths from all causes in 1 year *1000
No. of persons in the population at midyear
Uses of Mortality data
• Index of severity of disease
• Index of risk of a disease
1. When case fatality rate is high
2. When duration of disease (survival) is
short
Problems
• Underlying cause of death
• Quality of data
• Cross comparability over time
