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.”

MORBIDITY RATE:

The health workers are interested in assessing the health status of the communities through morbidity.morbidity relates to types and varieties of diseases one faces or experiences 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 cases of specific 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

- No loss to follow-up or change in smoking status over time

PREVALENCE

Prevalence rate related to the total number of cases,(i,e) new or old excisting

at a point of time with total population.

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,new or

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?

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 / n 2

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-

exposed which provides the risk difference

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%