This document discusses key concepts in statistical epidemiology including measures of disease frequency such as incidence rate and prevalence. It defines incidence rate as the number of new cases of a disease in a population over a time period, divided by the total population. Prevalence is defined as the total number of cases (new and existing) at a point in time, divided by the total population. Relative risk compares the risk of an event between exposed and unexposed groups, while attributable risk is the difference in risk between the two groups. Attributable fraction represents the proportion of disease cases among the exposed group that can be attributed to the exposure.

1. STATISTICAL EPIDEMIOLOGY
Epidemiology study of disease condition in population
Epidemiology is the science that studies the patterns ,causes,and effects of health and disease
conditions in defined the populations.it is the cornerstone of public health and informs policy
decisions and evidence-based practice by identifying risk factors for disease and targets for
preventive healthcare.
Fertility rate- study related to the birth
Mortality rate – study related to the death
Morbidity rate- study related to the disease condition.
OBJECTIVES OF EPIDEMIOLOGY
 To determine the rates of disease by person, place and time - Absolute risk ( incidence,
prevalence)
 To identify the risk factors for the disease - Relative risk (or odds ratio)
 To develop approaches for disease prevention - Attributable risk / fraction
MEASUREMENTS IN EPIDEMIOLOGY
 Epidemiology is about identifying associations between exposures and outcomes.
 To identify any association, exposures and outcomes must first be measured in a
quantitative manner.
 “measures of disease frequency.”
 “measures of association or effect.”
 “measures of potential impact.”

2. MORBIDITYRATE:
The health workers are interested in assessing the health status of the
communities through morbidity.morbidity relates to types and varieties of
diseasesone faces orexperiences affecting the day-to –day activities.data for
the study of morbidity of a community are available from registration
through hospitals
INCIDENCE RATE:
Incidence rate relates to the total number of new casesofspecific disease in
during year with mid year population.
 Incidence is defined as a number of new cases of a disease occurring in a specified time
period divided by the number of individuals at risk of developing the disease during the
same time
 Incidence is most relevant clinically
Total number of new cases of a specific disease during year
Incidence rate = × K
Mid year population
K is constant value,
The value of k may be taken as 100,10000 or 100000.base the numerator value we choose the
value of K
Example:
- 10,000 people without heart disease
- Follow population for 5 years
- 3,000 people smoke
- 410 of smokers develop heart disease
- 350 of non-smokers develop heart disease

3. - No loss to follow-up or change in smoking status over time
PREVALENCE
Prevalence rate relatedto the total number of cases,(i,e)new orold excisting
at a point of time with totalpopulation.
o Prevalence is defines as a total number of affected individuals in a population at a
specified time period divided by the number of individuals in the population at the
time
Total number of cases,newor
old existing at a point of time
Prevalence = ––––––––––––––––––––––––––––––– × K
total Population at a point of time
o That is, prevalence is equal to the probability of disease:
Mid year prevalence:
Arithmetic mean of the population on 1 jan and the population on 31 december of a
year.it is used to calculate annual rates.
TYPES OF PREVALENCE
1.POINT PREVALENCE
 It is relates to prevalence with respect to a specific point in time.
 Point prevalence is the proportion of a population that has the condition at a specific
point in time.
 Eg . Did you have an asthma attack on Monday?

4. 2.PERIOD PREVALENCE
 Related to prevalence over a defined period of time.
 Period prevalence is the proportion of a population that has the condition at some time
during a given period (e.g., 12 month prevalence), and includes people who already have
the condition at the start of the study period as well as those who acquire it during that
period.
 Eg . Did you have an asthma attack in january?
3.LIFE TIME PREVALENCE
 Lifetime prevalence (LTP) is the proportion of a population that at some point in their
life (up to the time of assessment) have experienced the condition
 Eg . Have you ever had an asthma attack
RELATIVE RISK OR RISK RATIO (RR)
In statistics and epidemiology, relative risk or risk ratio (RR) is the ratio of the probability of an
event occurring (for example, developing a disease, being injured) in an exposed group to the
probability of the event occurring in a comparison, non-exposed group.
 Categorical data in medical/health/nutrition investigations are frequently used to calculate
the risk of disease who are exposed to a specific character and not exposed to a specific
character.the basic requirement of this syudy is 2 × 2 table giving number of diseased
cases in those exposed and in those unexposed.
 Consider the first group as exposed and second group as exposed and second group as
unexposed.if n11 & n 12 get the disease in the respective groups.
Risk of the disease in the exposed group = n11 / n 1
risk of the disease in the unexpected group = n12 / n2

5. n11 / n 1
relative risk =
n12 / n 2
Relative Risk interpretation
 Ratios > 1.0 indicate rate is higher among exposed than unexposed (indicates positive
association)
 Ratios = 1.0 indicate no association
Ratios < 1.0 indicate rate is lower among exposed than unexposed( indicates negative
association)
ATTRIBUTABLE RISK (AR)
 In epidemiology ,the attributable risk is the difference in the rate of condition between the
risk in exposed subjects and unexposed subjects is called attributable risk
 This can also be understood as risk difference.
 Attributable risk is mostly calculated in cohort studies,where individuals are assembled
on exposure status and followed over a period of time.
 Attributable risk gives more valid information to the health managers than the relative
risk. This happens particularly when the disease is rare but occurs several times more
frequently if a particular antecedent is present.
 Attributable risk is better index of the public health importance of the risk factor in terms
of the impact its reduction can make on the overall incidence of disease.
 Calculation of AR = risk(incidence) in exposed – risk(incidence) in non-
exposedwhich provides the risk difference

6. ATTRIBUTABLE FRACTION (AF)
 AF is the proportion of disease incidence that can be attributed to a specific exposure
(among those who were exposed)
 AR divided by incidence in the exposed X 100%