Descriptive epidemiology
Introduction
As the term descriptive indicates, this type of study focuses on describing people who develop
disease in terms of their personal characteristics and where and when they were exposed to the
agent causing the disease. We commonly refer to these as person, place, and time variables.
Unlike analytical studies where you start with a hypothesis, in descriptive studies there are no
preconceived ideas about the relation between exposure and disease although the results from the
analysis can suggest hypotheses that can then be tested by analytical studies.Descriptive studies
stands in the first phase of an epidemiological investigation.
Definition
Epidemiology:The study of the distribution and determinants of health-related states in specified
populations, and the application of this study to control of health problems.
Descriptive Epidemiology :It is concerned with the distribution of disease, including what
populations do or do not develop a disease, in what geographical locations it is most or least
common, and how the frequency of occurrence varies over time.
Objectives of Descriptive Epidemiology
To evaluate trends in health and disease and allow comparisons among countries
and subgroups within countries.
To provide a basis for planning, provision and evaluation of services
To identify problems to be studied by analytic methods and to test hypotheses
related to those problems
Provides answers to the questions
WHAT – disease
WHO – person
WHERE – place
WHEN - Time
Aspects of Descriptive Epidemiology
TIME : It incudes
When does the disease occur commonly or rarely?
Is the frequency of disease at present different from the frequency in the
past?
The descriptive studies based on time can be divided into
Secular-Long time trend of disease occurrence or known as cyclic trends (recurrent
alterations in occurrence , interval or frequency of disease) which can be:
o Secular cyclicity
Levels of immunizations
Build up of susceptibles
e.g. Hep A-7 yr cycle,Measles-2yr cycle
o Short term cyclicity
Chickenpox, salmonella(yearly basis)
Periodic- Temporal interruption of the general trend of secular variation
Seasonal- A cyclic variation in disease frequency by time of year & season.
Seasonal fluctuations in,
Environmental factors
Occupational activities
Recreational activities
Epidemic- An increase in incidence above the expected in a defined geographic area
within a defined time period
PLACE
Where are the rates of disease highest or lowest?
The geographic area in which contact between the susceptible host and etiologic agent
potentially occurred. It can be:
School
Residence
Workplace
Municipality
Province
Country
Birthplace
District
City
Region
Restaurants
Convention or recreational sites
Recent travel history
PERSON
Who is getting the disease?
Organizing epidemiologic data according to the characteristics of the people involved
Demographic variables
Age
Gender
Race
Ethnic group
Income
Education
Occupation
Marital status
Personality type
Religion
Social status
Family variables
Family size
Maternal age
Mother’s education
Birth order
Parental deprivation
Other variables
Blood type
Past environmental exposures
Personality traits
Life style
Immune status
Vaccination status
Nutritional status
Exercise routine
Sources of information for descriptive epidemiology
Census data
Vital statistical records
Employment health examinations
Clinical records from hospitals
National figures on food consumption , medications, health events etc
Steps in descriptive epidemiology
A. Defining the population to be studied
B. Defining the disease under study
C. Describing the disease by time , place and person
D. Measurement of disease
E. Comparing with known indices
F. Formulation of etiological hypothesis
A. Defining the population to be studied
It is based in terms of number, age sex, occupation, cultural characteristics etc. The defined
population may be whole or sample of the population, specific groups like age and sex groups,
occupational groups , hospital patients, school children etc.
B. Defining the disease under study
After the population is defined, the disease must be investigated.
C. Describing the disease
The primary objective of descriptive epidemiology is to describe the occurrence and distribution
of disease by time , place and person and identifying those characteristics associated with
presence and absence of disease in individuals . This involves systemic collection and analysis
of data .
Time distribution
The pattern of disease may be described by the time of its occurrence i.e by week, month, year
, the day of the week hour of onset etc. There are three kinds of time trends of fluctuation in
disease occurrence(another classification apart from secular, periodic and seasonal)
Short term fluctuations.
eg: an epidemic like
a) Common source epidemics
i. Single exposure or point source epidemics
ii. Multiple exposure epidemics
b) Propagated epidemics
i. Person to person
ii. Arthropod vector
iii. Animal reservoir
c) Slow epidemics
Periodic fluctuations
a) Seasonal trend
b) Cyclic trend
Long term or secular trends-occurrence of a disease over a long period of time,
generally several years or decades.eg: Coronary heart disease, lung cancer
Place distribution
Studies of the geography of disease is one of the important dimensions of descriptive
epidemiology. It helps in knowing the disease pattern between countries and within countries.
The variations may be classified as
a) International variations: The pattern of disease varies from place to place. eg: The
cancer of the stomach is very common in Japan, but unusual in US.
b) National variations: For eg: The distribution of endemic goiter, leprosy, malaria have
shown variations in their distribution in India. It is measured in terms of incidence,
prevalence and mortality rates.
c) Rural-urban differences: For eg: Chronic bronchitis, accidents, lung cancer, drug
dependence are usually more frequent in urban than in rural areas.
d) Local distributions: Inner and outer city variations are well known. They are best
studied with the help of shaded maps. These maps show at a glance areas of high and
low frequency. Eg: A map showing ‘clustering’ of cases suggest a source of infection
shared by all the cases.
In short the geographic difference in disease occurrence is an important dimension of a
descriptive study and these differences are determined by the agent, host and environmental
factors. It provides evidence of the source of disease and its mode of spread.
Migration studies
They are carried out on two ways
a) Comparison of disease and death rates for migrants with their kin who have stayed at
home provides information on environmental variation on genetically similar groups.
b) Comparison of migrants with local population of the host country provides information
on genetically different groups.
Person Distribution
The host factors basic to epidemiological studies are:
1. Age: Certain diseases are most common in certain age groups than in others.eg: Measles
in childhood, cancer in middle age, atherosclerosis in old age.
2. Sex: It has been found that certain chronic disease such as diabetes , hyperthyroidism,
and obesity are strikingly more common in women than in men; and disease such as
lung cancer(4:1) and coronary disease are less frequent in women.
3. Ethnicity: Differences in disease occurrence have been noted between population
subgroups of different racial ethnic origin. It includes tuberculosis, hypertension,
coronary heart disease cancer and sickle cell anaemia.
4. Marital status: There are studies conducted on mortality in relation to marital status and
is found that mortality rates were always lower for married males and females than for
unmarried of the same age and sex. Also it is seen that cancer cervix is rare in nuns.
5. Occupation: Occupation alters habit pattern of employees.eg: sleep, alcohol, smoking,
drug addiction etc. People working in different areas are for different disease risks. eg:
coal mine workers develop silicosis.
6. Social class: Epidemiological studies show that health and disease are not equally
distributed in social classes. Individuals in the upper social classes have a longer life
expectancy and better health and nutritional status than those in lower social classes.eg:
in higher classes-coronary heart disease, hypertension and diabetes are common.
7. Behaviour: Human behavior is increasingly a risk factor for coronary heart disease,
cancer, obesity and accidents. The behavioural factors caught more attention are drug
abuse, cigarette smoking, sedentary life and over eating.
8. Stress: Stress have been shown to affect a variety of variables related to patients
response .eg: susceptibility to disease ,exacerbation of symptoms, compliance with
medical regimen.
9. Migration: Migration of people from rural to urban areas spread the infection from place
to place. Human movement may be classified as
short term, long term and permanent
According to age , sex, education and occupation
Internal or external
Urban versus rural
Thus study of host factors in relation to disease occurrence is an important dimension of
descriptive epidemiology.
D. Measurement of disease
It is done in the form of mortality , morbidity ,disability and so on and is done for
different subgroups of population
Mortality is done straight forward.
Morbidity has two aspects: incidence and prevalence
Incidence is obtained from longitudinal studies and prevalence from cross sectional
studies.
Longitudinal studies
Here the observations are repeated in the same population over a period of time by means
of follow up examinations .They are used to study
the natural history of a disease and its future outcome
for identifying risk factor of a disease to find out incidence rate
Cross sectional studies (prevalence studies):
Here there is single examination of a cross section of population at one point in time, the
results of which are projected on the whole population.Such studies tell us about the
distribution of a disease in population rather than etiology.
E. Comparing with known indices: By making comparisons between different
population and subgroup of the same population, it is possible to arrive at clues to
disease etiology We can also identify or define groups who are at risk for certain
disease.
F. Formulation of a hypothesis:
The study of the distribution of disease and utilizing the techniques of descriptive
epidemiology , it becomes easy to formulate the hypothesis relating to disease
etiology. The epidemiological hypothesis should specify the following
The population-the characteristics of the persons to whom the hypothesis
applies.
The specific cause being considered
The expected outcome-the disease
The dose response relationship-the amount of the cause needed to lead to a
stated incidence of the effect.
The time response relationship-the time period that will elapse between
exposure to the cause and observation of the effect.
Uses of Descriptive Epidemiology
Provide data regarding the magnitude of the disease load and types of disease load in the
community in terms of morbidity and mortality rates and ratios.
Provide clues to disease etiology and help in the formulation of etiological hypothesis.
Provide background data for planning, organizing and evaluating preventive and
curative services.
Contribute to research by describing variations in disease occurrence by time, place and
person.
