Rates calculation in relation to epid.ppt

Epidemiology lecture

1. RATES
By: Dada Robert
BPH, Dip CM&PH

2. Rate

A value describing one quantity in terms of
another.

A common rate type is a quantity expressed in
time, such as percent change per year.

Consider a population P with D disease
outcome (cases)

Rate = (D/P) x a constant.

(Constant = 1,000 or 10,000 or 100,000
people)

3. Types of rates
1.Crude rates: Apply to the total population
in a given area
2.Specific rates: Apply to specific subgroups
in the population (age, sex, etc) or specific
diseases
3. Standardized rates: used to permit
comparisons of rates in population which
differ in structure (e.g. age structure)
Two methods of standardization: Direct,
indirect

4. Morbidity rates
Morbidity rates are rates that are used to
quantify the magnitude/frequency of
diseases
There are two common morbidity rates

Incidence rates are (Cumulative incidence,
incidence density)

Prevalence (Period prevalence, point
prevalence)

5. Incidence

Number of new cases of a disease occurring in a
specified period of time.
(Number of new cases of specific disease during
a given period)/(population at risk during that
period) x 1000
Importance: If incidence increases, it may
indicate failure or ineffectiveness of control
measures of a disease and need for better/new
health control measure.

6. Prevalence

Number of current case (old and new) of a specified
disease at a point of time

It help to estimate the burden of disease

Identify potentially high-risk populations. They are
essentially helpful to plan rehabilitation facilities,
manpower needs, etc.
(Number of current case of a specified disease at a point of
time)/(estimated population at the same point of time) x 100

Point prevalence AND Period prevalence

7. Relationship between incidence and
prevalence

Prevalence =Incidence x Duration

8. Approach of an epidemiologist
Asking questions
making comparisons
● Asking questions may provide clues to cause or
aetiology of disease e.g.
What is the event,
what is its magnitude,
where did it happen,
when did it happen,
who were affected,
why did it happen?

9. Incidence rate
The proportion of a population that
develops a disease over time.
The risk/probability of an individual
developing a disease overtime.
The rapidity with which new cases of a
disease develop over time.
The proportion of unaffected individuals
who on average will contract the disease
over time.

10. Cumulative incidence
Number of new cases of a Cumulative
= disease during a specified period
Incidence Population at risk in the same
Period

11. Practical challenges in measuring
incidence rate
1.Identification of population at risk
The population at risk constitutes all
those free of the disease and susceptible
to it.
2. Population is not static/it fluctuates/as a
result of births, deaths and migration
3. People are at risk only until they get the
disease and then no longer at risk.

12. Practical solution to the challenges
1.Use the total population as a denominator
This gives an estimate of the incidence rate
and not the actual incidence rate.
2. Use person-time at risk
Incidence density=number of new cases of
a disease over a specified period/person-
time at risk.

13. Prevalence rate
It measures the proportion of a population
with a disease during a specified period
or at a point in time

Two types
1.Point prevalence rate
2.Period prevalence rate

14. Point prevalence rate

Measures the proportion of a
population with a disease at a point
in time.

Point prevalence rate=All persons
with a disease at a point in time/Total
population
It is not a rate, but a true proportion

15. Period prevalence rate

Measures the proportion of a
population with a disease in a
specified period.

Period prevalence rate = All persons
with a disease over time.
period/Average(mid-year)population
in the same period.

16. Incidence Vs prevalence
Incidence rate considers only new cases of a
disease
Prevalence rate considers all (new + old)
cases of a disease
Incidence rate considers population at risk
as a denominator
Prevalence rate considers total population
as a denominator

17. Commonly used specific rates
Relating to infants and children
Infant mortality rate = # of deaths in under 1 year old x 1,000
# of live births during the year
Post-neonatal mortality rate = # of deaths in infants between 1 month and 1 year x 1000
# of live births during the year
Neonatal mortality rate =
 
# of deaths in infants under 1 month of ages x 1,000
# of live births during the year

18. Specific rates relating to pregnancy and
puerperium
Stillbirth rate = # of deaths in 28 completed or more weeks of gestation x 1,000
# of live births plus stillbirths
Perinatal mortality rate = # of stillbirths + deaths under 1 week x 1,000
# of live births and stillbirths
Maternal mortality rate = # of M.D due to preg, childbirth, and puerperal conditions x 1,000
Total # of live births and stillbirths
Fertility rate = # of births in a year x 1,000
# of women of reproductive age15-49 years

19. Mortality rates
 These rates measure the magnitude of
deaths in a community.
Some are crude like the crude death rate.
Others are cause-specific mortality rate.
Some others are adjusted like standardized
mortality ration.

20. Common Mortality rates

Crude death rate

Age-specific mortality rate

Sex-specific mortality rate

Cause-specific mortality
rate

Proportionate mortality
ratio

Case fatality rate

Fetal death rate

Perinatal mortality
rate

Neonatal mortality
rate

Infant mortality rate

Child mortality rate

Under-five mortality
rate

Maternal mortality
ratio

21. Concept of Measuring Risk

To every disease event or outcome, there is a risk or
risks.

The risk of a disease event is measured in three ways:
i. Relative risk,
ii. Attributable risk,
iii. Absolute risk.

22. Relative risk (RR)

It is ratio of incidence of the disease among the exposed and
incidence among the non-exposed.
RR (incidence of disease among exposed)/
(Incidence of disease among non-exposed)
=a/(ab)/c/(cd)
=28/17.4
=1.6

If RR is more than 1, then there is a positive association

If RR is equal to 1, then there is no association

Smokers develop CHD 1.6 times more than nonsmokers.

23. Relative risk (RR)

Expresses risk of developing a diseases in exposed
group (a + b) as compared to non-exposed group (c
+ d)
RR= Incidence (risk) among exposed
Incidence (risk) among non-exposed
RR= a/(a+b)
c/(c+d)

24. Interpretation of relative risk
What does a RR of 2 mean?
Risk in exposed =RRX Risk in non-exposed
RR of 2 means
Risk in exposed=2X Risk in non-exposed
Thus a relative risk of 2 means the exposed
group is two times at a higher risk when
compared to non-exposed

25. Attributable Risk (AR)

AR indicates how much of the risk is due to
/attributable/ to the exposure
Quantifies the excess risk in the exposed that can
be attributable to the exposure by removing the
risk of the disease occurred due to other causes
AR= Risk (incidence) in exposed- Risk (incidence)
in non-exposed
AR= {a/(a+b)} / {c/(c+d)))}
Attributable risk is also called risk difference

26. Attributable risk (AR)

This is defined as amount or proportion of disease
incidence that can be attributed to a specific exposure.

It indicates to what extent the disease under study can be
attributed to the exposure:
(incidence of disease among exposed)
- (incidence of disease among non exposed)/
(Incidence of disease among exposed)

27. Example

The incidence of colon cancer disease
among cigarette smokers is 28.

The incidence of the same disease
among the non-smokers is 17.4

Calculate the attributable risk of the
disease to the population at risk.

28. Solution

AR= Risk (incidence) in exposed- Risk
(incidence) in non-exposed
AR=28-17.4/28
=10.6/28
= 0.379
= 37.9%

37. 9% of colon cancer disease among
the smokers was due to smoking.

29. Interpreting AR

What does attributable risk of 10 mean?

10 of the exposed cases are attributable to
the exposure.

By removing the exposure one can prevent
10 cases from getting the disease.

30. Attributable risk percent (AR%)

Estimates the proportion of disease among the
exposed that is attributable to the exposure.


The proportion of the disease in the exposed that
can be eliminated by eliminating the exposure

AR%=(Risk in exposed – Risk in non expose) X100%
Risk in non-exposed

31. Interpretation of AR%

What does AR% of 10% mean?

10% of the disease can be attributed to the
exposure.

10% of the disease can be eliminated if we
avoid the exposure.

32. Population Attributable Risk (PAR)

Estimates the rate of disease in the total
population that is attributable to exposure.

PAR = Risk in population – Risk in unexposed.

PAR = ARX prevalence rate of exposure.

33. Population attributable risk percent
(PAR%)


Estimates the proportion of disease in the
study population that is attributable to
exposure and thus could be eliminated if the
exposure were eliminated.

PAR%= Risk in population – Risk in unexposed
Risk in population

34. Possible outcomes in studying the
relationship between exposure &
disease
1.No association
RR=1
AR=0
2. Positive association
RR>1
AR>0
3. Negative association
RR<1 (fraction)
AR<0 (Negative)

35. Absolute Risk

Also called incidence

Incidence enables us to determine a person's
probability of being diagnosed with a disease during
a given time.

Hence, incidence is the number of new cases
diagnosed with a disease.

Because it deals with new disease events,
incidence is a measure of risk.

An incidence rate is the number of new cases of a
disease divided by the number of persons at risk for
the disease per a constant population.

36. Odds ratio

It is a measure of strength of association between the
risk factor and outcome.

The derivation of the odds ratio is based on three
assumptions:

●
The disease being investigated is relatively rare

●
The cases must be representative of those with the
disease

●
The controls must be representative of those without the
disease.

37. Odds ratio a.d/b.c
33X27/55X2 = 8.1

People who smoke less than 5 cigarettes per
day showed a risk of having lung cancer 8.1
times higher as compared to non-smokers.

38. 
OR is > 1- "those with the disease are more
likely to have been exposed,"
OR close to 1 then the exposure and disease
are not likely associated.
OR <1-exposure is a protective factor in the
causation of the disease.

39. Thanks for listening
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