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Epidemiologic disease
measurements
Measures of disease occurrence
• These are measurements of the disease frequency,
magnitude, and amount of disease in populations
• Diseases are measured by 4 quantitative descriptors
-Numbers
-Ratios
-Proportions
-Rates
Quantitative measurements
• Ratio: Is the expression of the relationship between two quantities
expressed as X:Y, where X is the count of one item and Y a count of
another. Ratio is used to show quantity of disease in a population
as: Cases divided by population. It is the basic measure of the
relative magnitude of two numbers. E.g., sex ratio = No of males/no
of females. Two types of ratios commonly used are; 1.Dimension
ratio, example: The # of hospital beds per 1000 population is a ratio
possessing dimension. 2.dimensionless ratio, example: Male to
female ratio is dimensionless.
• Proportion: Is the expression of the relationship of one part to the
whole. It is a dimensionless fraction quantity whose numerator is
included in the denominator. It can be expressed as a decimal (0.5),
or as a percentage (50%).
• When we call a measure a Ratio, we mean a nonproportional ratio.
• When we call a measure a proportion, we mean a
proportional ratio that doesn’t measure an event
overtime
• When we call a rate, we mean a proportional ratio
that does measure an event in a population
overtime.
Quantitative measurements cont.
• Since the base always equals 100, a proportion is expressed as
a percent. i.e. X\YxK. Where X is a count of one item in the
population. K, is the base number. The value of X and Y are
determined during the same time interval, e.g. 15 boys and 5
girls, the proportion of female is 5/20x100=25%
• The comparisons are achieved by looking at proportions, for
instance; The number of new TB cases in Morogoro region is
870, and if the population is 870,000. The proportion of new
TB cases is 870/870,000= 0.001, or 0.10% or 0.1/1000. This
proportion has to be compared with the # of new TB cases
from a different region to decide on which region has more
new Tb cases.
Quantitative measurements cont
Rate: Is the expression of the probability of occurrence of
a particular event in a population during a specified
period of time. It indicates the frequency of
events(births, illness, deaths) occurring in a population
per unit of time. The scale of the
measure(percent/1000 or per 100,000) should always
be stated, as well as the unit of time (per year, per
month, etc).
Mathematically X/YxK, where X is the number of events
or cases, Y is the total number of population at risk, and K
is a round number or base chosen to express the rate as a
number greater than one. The denominator must be
clearly defined and the numerator completely counted.
Quantitative measurements cont
• The values of X and Y are determined during
the same time interval.
• Rate is commonly used in epidemiology
because it most clearly expresses probability
or risk of disease or other events in a defined
population over a specified period of time.
Natality Rates
• Natality rates measures the rate of birth.
• Crude birth rate: is expressed as:
• Total of live births during given time interval
divided by estimated mid-interval population.
• It is expressed per 1000 population.
• Fertility rate: is expressed as:
• Total of live births during given time interval
divided by female aged 15-45years at mi-interval.
Natality rate cont.
• Fertility rate is expresses per 1000 population.
Morbidity rates
• Morbidity rate measures the rate of illness, it is a rate used to
quantify the magnitude of diseases.
(a) Incidence rate: Is expressed as; # of new cases of specific
disease during a given time interval over the estimated mid-interval
population at risk times K the base.
• Note: Incidence rate is a measure of the rate at which healthy
people develop disease during a specific time period, it is a
statement of probability, since incidence rates are affected by any
factor that affects the development of a disease, they can be used
to detect etiological factors and monitoring trends, also useful in
setting targets for improving health and measuring effectiveness of
treatments also used in the study of the efficacy of an intervention.
Incidence is a better measure than the prevalence
Morbidity rates cont.
• Incidence rate, used when examining an
outbreak of a health problem
b) Prevalence rate: Is expressed as; #of new and
old cases, divided by mid-interval population at
risk times K, k= could be per 100, per 1,000, per
100,000 depending on the frequency of the
disease.
• Number of current cases, i.e., people contracted
the disease before time period began and still
have the disease plus new cases during specified
time period is equal to prevalence.
Morbidity rates cont.
• Cumulative Incidence, refers to proportion of
individuals who become diseased during a specified
time period. Time period can be a calendar year, 6
months, 3 years, 5 years, etc.
Prevalence rate contains all known cases in the
numerator, it is used primarily to measure the
amount of illness in a community and thus, can be
used to determine the health care needs of a
particular community.
• Prevalence rates are influenced by both the
incidence of disease and by the duration of illness.
Therefore, P= I x D.
Morbidity rates cont.
• Types of prevalence:
• Point prevalence, is the # of cases that exist at
a given point in time. E.g., 1st June 1990.
• Life time prevalence, is the proportion of the
population that has a history of a given
disorder at some point in time.
• Period prevalence, refers to a given time
interval.
Relationship of incidence&
prevalence
• Relationship between Incidence and Prevalence
-If Incidence is low, but duration is long(chronic),
prevalence will be large in relation to incidence.
-If incidence is low due to short duration caused by
number of reasons, recovery, Improved reporting,
diagnostic facilities, migration, deaths, etc; prevalence
will be small in relation to incidence.
Prevalence provides information for, planning of drug
requirement, needed personnel, equipment needed,
e.g., beds, laboratory facilities etc. also information for
monitoring of disease control programs and for
tracking changes in disease pattern.
Special types of rates
• Attack rate: expressed in percentage as; #of new
cases of specific disease at specific time Interval
divided by total population at risk during same
time interval. Normally expressed as a
percentage.
• Mortality rate: #of deaths in a population at risk
during a specified time period. It is expressed as
the total number of deaths during given time
interval divided by estimated mid-interval
population, multiplied by K. K =1000.
Special Rates cont.
• Maternal mortality rate: expressed as number of
maternal deaths during given time interval divided
by #of child-bearing women aged 15-45 yrs during
the same time interval multiplied by K. K=100,000
live births.
• Case-fatality rate: expressed as number of deaths
assigned to specific disease divided by number of
cases of the disease multiplied by K. K=100.
Special rates cont.
• Crude death rate: expressed as total #of deaths
during a given time-interval divided by estimated
mid-interval population multiplied by K. k=1000
• Specific rates. Are calculated for various
segments of the population. The rates are
difficult to calculate because more information
about the demographic composition of the
population must be known than with other rates.
Special rates
• Cause-specific death rate: expressed as
number of deaths assigned to specific cause
during given time interval divided by
estimated mid-interval population multiplied
by K. k=100,000 population.
• Adjusted rates: Equalize the differences in the
population at risk, so the rates are
comparable.
Special rates
• However, adjusted rates are difficult to
calculate because the demographic
composition of the population must be
known.
• Age-adjusted rates are most frequently used
to compare mortality in different populations.
Are computed either by direct or indirect
method.
Measures of Association
• Are calculations used to measure disease
frequency relative to other factors.
• Are also indications of how more or less likely
one is to develop disease as compared to
another.
• These are:
• Risk ratios, Odds ratios, Rate ratios, Relative
risk ,etc.
Measures of association cont.
• A risk ratio greater than 1, implies that the risk
of disease is higher in the exposed group than
in unexposed group, while a risk ratio less
than 1 occurs when the risk is lower in the
exposed group, suggesting that the exposure
may be protective.
• A risk ratio of 1 occurs when risks are the
same in two groups. No association between
exposure and the disease.
Measures of association cont.
• Risk ratio is expressed as risk in exposed group
divided by risk in unexposed group
• Rate ratio is expressed as rate in exposed group
divided by rate in unexposed group
• Odds ratio Is a measure of effect size that describes
the strength of association or a non- independence
between 2 binary data values. In statistics, the odds
of an event occurring is the probability of the event
divided by the probability of an event not occurring.
Think of how likely will the event occur compared to
how likely will the event not occur?
Measures of association cont.
• The risk and odds ratios can both be estimated from
longitudinal and from cross-sectional studies with
complete follow-up, for example:
• A vaccine which is believed that offers full protection
to some individuals but none to others, rather than
partial protection to all.
Measures of association
• Odds Ratios
• An odds ratio of 1 occurs when the odds, and
hence the proportions, are the same in two
groups and is equivalent to no association
between the exposure and the disease.
• The odds ratio is always further away from 1
than the corresponding risk(or prevalence)
ratio. Thus:
Measurement of association cont.
• If Risk Ratio greater than 1 then Odds Ratio is
greater than risk ratio.
• If Risk Ratio less than 1 then Odds ratio is less
than risk ratio
• The Odds ratio for exposure; i.e., the odds of
disease in the exposed compared to the odds in
unexposed group, equals the odds ratio for
disease, that is the odds of exposure in the
disease compared to the odds in healthy group.
Measurement of association cont.
• Attributable Risk: The risk ratio that assesses
how much more likely, for example, a smoker
is likely to develop lung cancer than nonsmoker, but it gives no indication of the
magnitude of the excess risk in absolute
terms. It is measured by Attributable risk.
• Attributable risk(AR)= risk among exposedrisk among unexposed=risk difference
Measures of association
• Relative Risk
• Measure of association between incidence of
disease and factor being investigated
• Ratio of incidence rate for persons exposed to
incidence rate for those not exposed.
• Estimate of magnitude of association between
exposure and disease.
• Infant Mortality rates (IMR)
• It is defined as the death of babies under 1
year per 1000 live births. The IMR data is
acquired through vital statistics, household
survey, health service records and census.
• Expressed as number of deaths of babies
under 1 year divided by total number of live
births during the year multiplied by K
constant.
• Maternal mortality ratio
• Expressed as X/Y; Number of maternal deaths
related to pregnancy divided by the number of
live births in a year multiplied by K.
• K=100,000 live births.
Sources of routine morbidity
statistics.
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Admission registers, records
Clinical records
Discharge summaries
Investigation laboratory records
Investigation request forms
Burial permits and other mortuary records.
Registers for birth malformations, accidents,
toxicity and other infectious diseases can be
established if are not currently in use.
Records of special health programs
1. Essential Drug Programs (EDP)
The EDP maintains the records of drug supplied,
how long they last, how many patients were
treated, and their respective health problems. It
is a good indicator of morbidity pattern at the
community level.
2. MCH(RCH) . The special cards are available for
the mother and the child, the state of health,
type of treatment procedure for each mother
and child is recorded regularly.
records of Special health programs
cont.
• 3. Expanded program on Immunization.
Generate and maintain information on all
vaccinations done, type of vaccination and their
batch numbers. Provide information mainly on
vaccination coverage.
4. Mental health
Maintains records of people who are mentally
ill.
Cont.
• 5.Oral and dental health.
• Maintains records of people found with or developing
oral or dental health problems.
Other record sources
Health requirements. Recruitment to new employment
require someone to undergo medical examination prior
to being recruited.
Disease screening programs. Screening of diseases in
communities as a basis of evaluation and disease
control programs. Census also collects information on
people who are sick, disabled or compromised quality
of living.
Determining Incidence
Consider;
• The method of classifying individuals as having
disease or not must be clearly stated, i.e. whether it
was by detailed examination, screening, or
examination of routine sources of health statistics.
• The date of onset, thus identifying incidence cases
from prevalence
• Denominator must be clearly defined and
enumerated
• Period of observation must be long enough for
the numerator to stabilize
• The denominator of the mid-year population
which should be enumerated during the
period of the study.
Measures of mortality of the fetus or
infant in use
Measure
IMR
Perinatal MR
Neonatal MR
PostNMR
ChildhoodMR
StillbirthRate
Early NMR
Late NMR
Fetal deathRate
time interval
birth-1yr
28wks gest-7dys
birth-28dys
28dys-1yr
1yr-5yrs
28wks gest.-birth
birth-7days
7dys-28dys
conception-birth
Denominator
Live births
Live births +stillbirths
live births
live births
live births
live births+stillbirths
live births
live births
live births+stillbirths
Other measures of health.
• Secondary attack rate, expressed as:
#of exposed people developing the disease of interest
within the range of the incubation period divided by
the total number of susceptibles exposed to the
primary case multiplied by 1000.
• Case fatality Rate is expressed as:
# of deaths assigned to a specific disease divided by the
number of cases of the disease. It is frequently
expressed as a percentage, it predicts the risk of
dying if the disease is contracted.