4Estimating Disease Frequency occurrence-2024.ppt

Lecturer: Ms Eleanor Turyakira
Email: eturyakira@must.ac.ug
MUST, FoM, 2023/2024
FUNDAMENTALS OF EPIDEMIOLOGY
Estimating Disease Frequency:
Incidence and Prevalence

REM the definition of Epidemiology:
The study of the distribution and
determinants of disease frequency in
human populations (Hennekens CH et
al, 1987)

Disease frequency in populations
 How many people have the disease of
interest during a specified period?
(Preexisting and New cases)
 How many people develop the disease of
interest during a specified period?
(New cases)
How many refers to Frequency count
Relative Frequency commonly calculated and
presented such as Proportion, Rate, Ratio

• Relative disease frequency measures consider:
– The number of individuals with the disease or
event of interest in the population (the count)
– The size of a define population. e.g
population defined by administrative
boundaries like a city. Often a combination of
place and person characteristics are used to
define the population.
– The time period e.g Jan 1st to Dec 31, 2021
Measures of Disease Frequency

Review of general measures of relative
frequency:
Proportion =A/N =A/(A+B)
o A is the number of persons with the disease or event of
interest.
o B number of people without the disease or event of interest
o N is the population size (equal to A+B)
o Numerator (A) is part of the denominator (N)
o Proportion often expressed as percentage i.e. A/N*100
thus values range from 0% - 100%.

frequency:
Rate=A/Time
• A is the number of persons with the disease or event of
interest.
• The denominator for rate calculation is time. Time units
may be hours, days, weeks, months, year etc
• The Rate is measures how fast disease is occurring in a
population.
• Values of Rates range from 0 to infinity

frequency:
Odds=A/B
oA is the number of persons with the disease or event of
interest.
oB number of people without the disease or event of interest
oOdds measure the likelihood of occurrence of disease in a
specified population.
Similarly, Odds=P/(1-P)
o P=proportion of persons with disease of interest
o (1-P)= proportion without the disease of interest
The values of Odds range from 0 to infinity

Basic Epidemiologic Measures of
Disease Frequency
o Prevalence
o Incidence

Prevalence of Disease
Prevalence is a measure of existing cases of
disease in a defined population. Prevalence is
expressed as a proportion.

Point prevalence: proportion of the population who have
disease at a particular point in time. Most measures of
prevalence are point prevalence.
Period prevalence: proportion of the population that had
the disease at some time during a specified calendar
period (usually one year).
Lifetime prevalence: proportion of a population that has
experienced a particular disease at some point in their
life.
Contact prevalence: proportion of the population with at
3 Types of Prevalence

Prevalence in a population
Has disease No disease Row totals
A B A+B=N
Everyone in the numerator is
included in the denominator.

Prevalence in a sample
Has disease No disease Row totals
a b a+b=n
Everyone in the numerator is
included in the denominator.

Incidence of Disease
Incidence is the number of new cases of
disease that develop in a population at risk
during a specified time period.

Incidence has three components
– New disease events.
– Population at risk : members who do not have
the disease at beginning of follow-up period,
but should be susceptible to the disease.
– Follow-up period, during which new disease
events occur and are recorded.
Measures of Incidence

Incidence proportion
• Risk
• Cumulative incidence
• Attack rate
Incidence rate
• Incidence density
• Force of morbidity or force of
mortality
• Hazard rate
Measures of Incidence

Incidence proportion in a sample
Developed
Disease
Did not develop
disease
Row
totals
a b n
Incidence Proportion: a/n = a/(a+b)
Numerator (a) are new cases of disease that developed during
the follow-up period.
Denominator (n) are people at risk of developing the disease of
interest during follow-up period.
Incidence proportion (Cumulative incidence) usually increases
over time, therefore time period must be specified.

Incidence: Attack Rate
The term attack rate is used to describe incidence in
outbreak situations.
Numerator=new cases;
Denominator=Total population at risk of the disease during a specified
period of time.
Attack rate is a measure frequency of morbidity and
speed of spread of a disease.
In outbreak surveillance, attack rate is monitored over
time to check the speed at which new infections are
occurring given the response/control measures
instituted.

Ebola virus disease outbreak in DRC by date of illness onset; August 2018 to Nov 2019.
Source: https://www.cdc.gov/mmwr/volumes/68/wr/mm6850a3.htm#F1_down

What is the difference between
incidence and prevalence?

Incidence
Recovery Death
Prevalence

Does an increase in prevalence
of disease in a population
mean that there is an increase
in incidence of disease in the
population?

Source: https://bmcpublichealth.biomedcentral.com/articles/10.1186/1471-2458-13-318
Age group (yrs)

When is prevalence or incidence an appropriate
measure of disease frequency?
• Is prevalence rate a useful measure of the frequency of
type 2 diabetes in different populations?
• What are the possible explanations for the variation in
diabetes prevalence rates indicated in graph?
• Is incidence an appropriate measure of frequency of
covid19 in different populations?
• What are the possible explanation for the variation in
covid19 incidence rates in different countries
(populations)?

• What factors could lead to increase in
prevalence despite low incidence?
• What factors could lead to a decrease
in prevalence despite high incidence?
Relationship between
Incidence and Prevalence

Measures of Mortality
Mortality rates are a special type of incidence rate
because death is always a “new event”.
• Crude death rate
• Cause-specific mortality rate
• Case-fatality rate
• Proportionate mortality rate
• Age-specific mortality rate
Death rates are useful for investigating diseases with
high case fatality rates.

(i) Crude mortality rate
(ii) Cause-specific mortality rate
Annual mortality rate
for all causes
(per 1000 population)= Total number of deaths from all causes in one year
Mid-year population at risk
X 1000
Annual mortality rate
from a specific cause =
Number of deaths from a specific cause e.g. lung
cancer in one year
Mid-year population at risk
X 1000

(iii) Case-fatality rate
(iv) Proportionate mortality(e.g in 2019)=
Case fatality rate
(percent) =
Number of individuals diagnosed as having a
certain disease who die of the disease during a
specified period of time
Number of persons with the specified disease
during the specified period of time
X 100
Number of deaths from a specific cause in 2009
Total deaths in 2009
X 100

Examples of Age-specific Mortality Rates
• Infant Mortality Rate: the risk of dying during the first
year of life among babies born alive (aged 0-365 days)
• Neonatal Mortality Rate: measures the risk of dying
within the first 28 days of life among babies born alive.
• Perinatal Mortality Rate: measures risk of dying
between 28 weeks gestation to 7 days after birth.
Perinatal deaths include still births (late fetal deaths)
and newborn deaths within the first week of life.

Burden of disease: What causes the most deaths?
Uganda
Canada
Source:
https://www.healthdata.org/

What drives the
most deaths and
disability?
Uganda
Canada
Source:
https://www.healthdata.org/

What challenges might researchers face when
investigating and comparing incidence of
disease over time or between regions?

Challenges in Determining Incidence /Prevalence
Measures
• Differences in case definition – the number of cases
(numerator) may vary depending on the diagnostic
criteria.
• Sources of study population: hospital records compared
to community surveys lead to variation in estimates of
epidemiologic measures of disease.
• Selection bias – hospital-based surveys in self-selected
populations who may differ from general population in
important ways
• Measurement bias: Incompleteness of data in hospital
records

Measures of disease frequency- Summary
• Measures of disease frequency are used to summarise
data and describe the distribution of health-related
events (morbidity and mortality).
• Morbidity data more useful when case-fatality of
disease is low, but disease affects many people e.g.
mental illness, musculoskeletal diseases, chickenpox.
• Morbidity data are useful in identifying trends
• Mortality data useful for investigating diseases with
high case fatality rates
• Both morbidity and mortality data are useful in
evaluating interventions

4Estimating Disease Frequency occurrence-2024.ppt

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