Statistical methods in epidemiology.- Epidemiology Incidence Prevalence Relative risk Attributable risk

1. By:
Ravinandan AP
Department of Pharmacy Practice
Sree Siddaganga College of
Pharmacy, Tumkur
Statistical
Methods In
Epidemiology

2. Presentation Outlines….
1. Epidemiology
2. Incidence
3. Prevalence
4. Relative risk
5. Attributable risk

3. Epidemiology
 It is the study of disease occurrence
& transmission in a human
population.
 It focus on the distribution &
determination of disease.
 The branch of medical science dealing
with the transmission and control of
disease
 It may also be considered as method
of public health-a scientific approach
to studying disease & health
problems.

4. Example for Epidemiology

5.  It consists of research methods &
specific strategies for counting &
calculating the occurrence & risk of
disease.
 Therefore, epidemiological studies of
drug use employ these methods &
statistical measures to study the
occurrence & distribution of drug use
& its associated problems.

6. Risk Assessment
 Risk measures are estimated that
describe the amount of risk
associated with a particular
exposure in a sample population.
 Risk estimates can quantitatively
describe the risk associated with
particular exposure & the
development or prevention of
disease, & they can quantify the
association between the exposure to
a particular to drug & an ADR.

7.  Risk estimated are part of our daily
lives.
 Measures of risk are communicated
to patients via newspapers, TV, and
the internet daily.
 Risk measures are communicated to
GP’s via studies published in
medical journals.
 These risk measures become
important in the clinical decision
making process for both patients &
the practitioners.

8. Prevalence
 It is defined as the number of
existing cases of disease (or any
outcome, ex ADR, drug use) in a
population at a particular point in
time.
number of existing cases in a
population
 P=--------------------------------------------
total number of people in that
population

9. Example:1
 On January 1, 2018 SSCP had
3000students enrolled. On January
1, 2018, 300 students at SSCP
reported dengue.
number of existing cases in a population
 P=--------------------------------------------
total number of people in that population
 P=300/3000=0.10 or 10%
 Therefore, on Jan 1, 2018, 10% of
the students at SSCP suffered from
Dengue.
 In other words, the prevalence of
dengue on Jan 1, 2018, was 10%

10.  Prevalence = no. of cases /
population size
 Prevalence can be measured in an
closed cohort or in an open
population.
 Prevalence in cross-sectional.
 “Old” cases and “new” cases are
counted in the numerator.
 Can be measured at a particular
point (point prevalence) or over a
period (period prevalence).
 Normally, when we say prevalence
we mean “point prevalence.”

12. Incidence
 It is defined as the number of new
cases of disease that develop in a
population at risk over a specified
time period.
 It is used to determine how often the
disease is occurring.
 It is typically described as either
cumulative incidence (CI) or an
incidence rate (IR).

13. Cumulative Incidence (CI)
 Cumulative incidence assumes that
all of the subjects were followed for
the entire study period.
 CI does not reflect study dropouts or
losses to follow - up.
number of new cases disease during
given time
 CI=--------------------------------------------
total population at risk

14.  Using SSCP college, in the first ex-1
the time frame of Jan 1, 2018,
through Dec 31, 2018, 30 students
developed dengue.
 CI= 30/3000-300= 0.011 or 1.1% per
year.
 For the time period from Jan 1, 2018,
through Dec 31, 2018, 1.1% of students
at SSCP developed dengue. Incidence of
dengue in SSCP was 1.1%

15.  Incidence measures the risk of
developing the disease or outcome
of interest in a population at risk;
therefore, any subjects who currently
have the disease or outcome of
interest at the onset of study period
are subtracted from the
denominator.

16. Incidence rate (IR)
 Also referred to as incidence density
(ID).
 It is a more accurate means of
measuring disease occurrence.
 IR takes in to account the actual
observation time of each subject
during the study period, rather than
assuming that all subjects were
followed for the entire period of time.

17. number of new cases
 IR=---------------------------------------
total person –time of observation
in population at risk

18.  A tool used to calculate risk estimates is
contingency table.
 These tables are used to analyze discrete
data.
 Incidences and prevalence are often
reported with a population multiplier such
“per m people” or “per m person-years.”
 To convert a rate or proportion to “per m
people,” simply multiplying by m.
 For example, an incidence rate of
0.00877 per person-year =0.008770 ×
100,000 = 877 per 100,000 person-years.

19. Definition of terms
 Probability: is the proportions (%) of
times an event would occur if an
observation was repeated many
times.
 Risk: is the probability of an event
among those experiencing the event
divided by the number who could
experience it (at risk)
 Odds: probability of an event divided
by the probability of the event not
happening

20. Relative Risk (RR)
 RR is the likelihood of
developing disease in the
exposed group relative to the
unexposed group; it is a
measure of association between
the exposure and the disease.
 RR = CI exposed /
CI unexposed

21.  RR can be estimated from a 2 X
2 contingency table.
 Interpretation of RR
RR Association
between exposure
& outcome
1 No association
<1 Negative association
/ Decreased risk
>1 Positive association
/
Increased risk

22. Attributable Risk (AR)
 It is also called risk difference (RD).
 Is another measure of risk used in
studies.
 It provides information on the
absolute effect of the exposure.
 It describes the excess risk of
disease in the exposed compared
with those who were unexposed.

23. ABSOLUTE RISK (AR)
 It is the probability that an
individual will experience the
specified outcome during a
specified period.
 It lies in the range 0 to 1, or 0%
to 100%.
 In contrast to common usage,
the word risk may refer to
adverse events (such as
myocardial infarction), or
desirable events ( such as cure)

24. Absolute risk reduction (ARR)
 It is the absolute difference
in risk between the
experimental and control
groups in a trial.
 It is used when the risk in
the control group exceeds
the risk

25.  AR= CI exposed –
CI unexposed
 It allows the investigator to
determine how morbidity &
mortality are affected by
removing exposure.

26. Attributable Risk percent
 The attributable risk can then be
converted to the (AR%).
 It makes easier to interpret.
 AR% =
CI exposed- CI unexposed X 100
CI exposed

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