DESCRIPTIVE STUDIES
Dr. A.K.AVASARALA
MBBS, M.D.
PROFESSOR & HEAD
DEPT OF COMMUNITY
MEDICINE & EPIDEMIOLOGY
PRATHIMA INSTITUTE OF
MEDICAL SCIENCES,
KARIMNAGAR, A.P.
INDIA : +91505417
avasarala@yahoo.com
LEARNING OBJECTIVES
LEARNER SHOULD BE ABLE TO KNOW AT
THE END OF THIS LECTURE
1. WHAT IS MEANT BY DESCRIPTIVE
STUDIES?
2. WHAT TYPES OF DESCRIPTIVE STUDIES
ARE AVAILABLE?
3. WHAT COMPONENTS OF THE DISEASE
AND POPULATIONS ARE TO BE
DESCRIBED?
PERFORMANCE OBJECTIVES
STUDENT SHOULD BE ABLE TO
1. CHOOSE THE SUITABLE DESCRIPTIVE
STUDY FOR THE DISEASE AND
POPULATION
2. Describe any health problem in terms of
time, place and person
3. Interpret his findings to formulate a
worthy hypothesis
DESCRIPTIVE STUDIES
THEY ARE SO CALLED BECAUSE THEY
DESCRIBE ALL ASPECTS OF
1) THE DISEASE AND
2) THE POPULATION AFFECTED
PURPOSE
TO FORMULATE
WORTHY HYPOTHESES
ULTIMATELY FOR TAKING
HEALTH ACTION
UNIT OF THE STUDY
THESE DESCRIPTIVE STUDIES
ARE MAINLY CARRIED OUT IN
POPULATIONS OR ITS SUB
GROUPS, NOT ON INDIVIDUALS
NATURE OF THE STUDY
OBSERVATIONAL IN NATURE
IN DESCRIPTIVE METHOD, WE
JUST OBSERVE THE EVENTS
OCCURRING AND WE DO NOT
INTERVENE,
HENCE ALSO CALLED
OBSERVATIONAL STUDIES
TYPES
ECOLOGICAL OR
CORRELATIONAL STUDIES
CROSS-SECTIONAL OR
PREVALENCE STUDIES
LONGITUDINAL OR INCIDENCE
STUDIES
ECOLOGICAL STUDIES-1
STUDIES CONDUCTED IN SPECIFIC
POPULATION GROUPS (E.G. MUSLIMS,
CATHOLICS, JEWS ETC) HAVING
SPECIFIC CHARACTERISTICS IN A
SPECIFIED GEOGRAPHICAL AREA
ECOLOGICAL STUDY METHOD
CAUSES OR RISK FACTORS ARE
STUDIED WITH REGARD TO THE
DISEASES AND DEATHS
OCCURRED IN A PARTICULAR
POPULATION.
BOTH ARE LINKED TOGETHER AND
THEIR CO-OCCURRENCE
(CORRELATION) IS ESTABLISHED IN
THESE STUDIES FOR HYPOTHESIS
FORMATION.
ECOLOGICAL CORRELATION
FREQUENCY
AND TRENDS
CORRELATED
OF THE
WITH
SUSPECTED
CAUSES/
RISK FACTORS
PREVALENCE
OR INCIDENCE
OF THE
DISEASE
OCCURING IN A
PARTICULAR
AREA
NON- SENSE CORRELATION
THIS STUDY OF CO-OCCURRENCE
DOES NOT SPEAK OF THE
CAUSAL ASSOCIATION ALWAYS
ONE MUST BEWARE OF
NON- SENSE CORRELATION
ADVANTAGES AND DISADVANTAGES
ADVANTAGES:
CONDUCTED AT GROUP LEVEL, NOT AT
INDIVIDUAL LEVEL, HENCE RELATIVELY EASY
TO DO AND QUICK
USE EXISTING DATA
GENERATE AND SUPPORT NEW HYPOTHESES
ECOLOGICAL STUDIES CONDUCTED OVER
TIME ON A SPECIFIC GEOGRAPHICAL AREA
ARE MORE CONVENIENT TO PERFORM AND
FORM HYPOTHESES RATHER THAN
STUDYING WHOLE POPULATIONS OR ITS
SAMPLES AS DONE IN DESCRIPTIVE
STUDIES.
ECOLOGICAL FALLACY
ECOLOGICAL FALLACY IS AN
ERROR IN INFERENCE THAT
OCCURS WHEN ASSOCIATION
OBSERVED BETWEEN
VARIABLES OF A GROUP LEVEL,
IS ASSUMED TO EXIST AT AN
INDIVIDUAL LEVEL.
EXAMPLES OF ECOLOGICAL
STUDIES
CANCER CERVIX IS RARE IN
JEWISH WOMEN DUE TO MALE
CIRCUMCISION
SICKLE CELL DISEASE IS MORE
SEEN IN INDIAN TRIBES
CROSS-SECTIONAL STUDIES
(PREVALENCE STUDIES)
THESE ARE SO CALLED BECAUSE A
CROSS SECTION OF A COMMUNITY
(FREQUENTLY TOTAL POPULATION
SAMPLES) IS STUDIED AT A
PARTICULAR POINT OR PERIOD OF
TIME.
NATURE OF THE STUDY
THEY CAN BE OF DESCRIPTIVE
NATURE WHEN ONE VARIABLE OR
EACH VARIABLE IN A GROUP OR
POPULATION IS STUDIED
OR
OF ANALYTICAL TYPE AS THEY ARE
SOUGHT TO PROVIDE INFORMATION
ABOUT THE PRESENCE AND
STRENGTH OF ASSOCIATION
WHEN TO CONDUCT
PREVALENCE STUDIES?
THEY ARE CONDUCTED FOR
CHRONIC DISEASES HAVING HIGH
PREVALENCE WITH VERY LOW
INCIDENCE.
PREVALENCE STUDY
METHODOLOGY-1
IT CAN BE DONE AT A SINGLE POINT OF
CALENDAR TIME (POINT PREVALENCE)
WHEN THE MEASUREMENT OF CAUSAL
RELATIONSHIP RELATE TO THE SAME
POINT IN STUDY MEMBERS’ LIVES
OR
CAN BE COMPLETED IN FEW MONTHS
OR YEARS (PERIOD PREVALENCE).
PREVALENCE STUDY
METHODOLOGY-2
THE DESCRIPTIVE INFORMATION IS
OBTAINED BY MEANS OF DOOR TO
DOOR SURVEY.
THOUGH THEY ARE USUALLY
CARRIED OUT ON POPULATIONS OR
SAMPLES OF POPULATION, THEY ARE
INDIVIDUAL BASED i.e. THEY SEEK
THE INFORMATION OF ABOUT THE
INDIVIDUALS IN A GROUP OR
POPULATION.
RAPID METHODS OF
PREVALENCE STUDIES
CLUSTER SURVEYS
COMPUTER SIMULATION
RANDOM DIGIT DIALING FOR
SAMPLING
COMPUTER BASED INTERVIEWS
& USE OF SPREADSHEET
PUBLIC HEALTH USES
WHEN THE CROSS-SECTIONAL
STUDIES ARE REPEATEDLY
DONE, THEY WILL SERVE THE
PURPOSE OF HEALTH AND
DISEASE SURVEILLANCE OF
THE POPULATION.
DISADVANTAGES
1. TIME CONSUMING AND EXPENSIVE
2. SUBJECTED TO RECALL BIAS AND
CONFOUNDING BIAS
3. OTHER COMMON BIASES
ENCOUNTERED IN THESE STUDIES
ARE OF BERKESONIAN, INFORMATION
AND INVESTIGATOR TYPES
4. UNDER- REPRESENT PEOPLE WITH A
SHORT-COURSE OF DISEASE
5. LIMITED TO STUDIES OF CAUSES THAT
ARE OF LONG STANDING NATURE
LONGITUDINAL STUDIES
IN A LONGITUDINAL STUDY DESIGN,
THE STUDY IS CONDUCTED AT
TWO OR DIFFERENT POINTS OF
TIME IN THE LIFE TIME OF
INDIVIDUALS UNDER STUDY IN
CONTRAST TO THE ONE TIME
STUDY OF CROSS-SECTIONAL
DESIGN.
METHODOLOGY
DEFINING AND DESCRIBING THE
POPULATION AFFECTED
(REFERENCE POPULATION AND STUDY
POPULATIONS)
DEFINING AND DESCRIBING THE
DISEASE
OPERATIONAL CASE DEFINITIONS
TIME, PLACE AND PERSON TRENDS)
FORMULATION OF HYPOTHESIS
POPULATION AT RISK
REFERENCE POPULATIONS
THE POPULATION WHICH IS AT RISK AND
THE HEALTH ACTION HAS TO BE INITIATED
AFTER THE STUDY IS OVER IS THE
REFERENCE POPULATION
OR
THE POPULATION IN WHICH A PARTICULAR
DISEASE OR EXPOSURE HAS OCCURRED
AND IS TO BE INVESTIGATED
REFERENCE POPULATION IS THE
ONE, WHICH HAS TO BE GET
BENEFITED AFTER OUR DESCRIPTIVE
STUDY.
STUDY POPULATION
THIS IS THE POPULATION ON
WHICH THE HYPOTHESIS IS
ACTUALLY STUDIED AND
TESTED.
IT MAY BE THE ENTIRE REFERENCE
POPULATION ITSELF (IF IT IS SMALL
AND FEASIBLE FOR STUDY)
OR
A REPRESENTATIVE PORTION OF
IT(SAMPLE).
DESCRIBING THE STUDY
POPULATION
AGE COMPOSITION,
SEX COMPOSITION,
OCCUPATIONS,
SOCIOECONOMIC STATUS,
LITERACY PROFILE
SOCIAL CUSTOMS, HABITS
SPECIFIC LIFESTYLES
KNOWLEDGE OF HEALTH
FACILITIES AVAILABLE AND THEIR
UTILIZATION
VITAL REQUIREMENTS OF
STUDY POPULATION
1. ITS REPRESENTATIVENESS TO THE
PARENT REFERENCE POPULATION
2. ITS OPTIMUM SIZE
THEN ONLY THE RESULTS OF
HYPOTHESIS TESTED ON ANY
STUDY POPULATION CAN BE
GENERALIZED TO THE REFERENCE
POPULATION.
DEFINING AND DESCRIBING
THE DISEASE
DEFINING THE DISEASE
(CASE DEFINITION)
AN OPERATIONAL WORKING
DEFINITION TO MAKE UNIFORM AND
UNBIASED COUNTING IN
POPULATIONS
TIME DISTRIBUTION
EPIDEMIC
ENDEMIC
SPORADIC
CYCLICAL TRENDS
SEASONAL TRENDS
SECULAR TRENDS
PLACE DISRIBUTION
INTERNATIONAL VARIATIONS
NATIONAL VARIATIONS
URBAN-RURAL COMPARISONS
CLUSTERING OF DISEASE
DISEASE MAPPING
PERSON DISTRIBUTION
AGE INFLUENCE
RACE, RELIGION & ETHNICITY
MARITAL STATUS
SOCIOECONOMIC STATUS (SES)
MIGRATION
PERSONAL HABITS
LIFESTYLES
APPLICATIONS
1. TO FORMULATE CAUSAL
HYPOTHESIS.
2. INDICATE THE DISEASE LOAD AND
FREQUENCY ALTERATIONS AND
THEREBY HELP TO MAKE FUTURE
PROJECTIONS.
DIAGNOSING AND TELLING THE
PROGNOSIS.
JURISDICTION
THE DIVIDING LINE BETWEEN THE
DESCRIPTIVE AND ANALYTICAL
STUDIES IS NOT VERY SHARP
ECOLOGICAL AND INDIVIDUAL
LEVELS OR CONTEXTS
THE HYPOTHESIS TO BE STUDIED
WILL BE MORE MEANINGFUL
WHEN IT IS APPLICABLE AT BOTH
ECOLOGICAL AND INDIVIDUAL
LEVELS OR CONTEXTS AND
ALSO EXPLAINS THE
ECOLOGICAL (GROUP)TO
INDIVIDUAL CORRELATION.
FORMULATION OF HYPOTHESIS
SOURCES TO FORM HYPOTHESIS?
1) DESCRIPTIVE STUDIES
2) ECOLOGICAL STUDIES ON SPECIFIC
GROUPS
3) KEEN OBSERVATION OF THE
DATA/INFORMATION AVAILABLE
4) INDUCTIVE REASONING (MILL’S
CANNONS)
5) DEDUCTIVE REASONING
DATA AND HYPOTHESIS
KEEN OBSERVATION OF ANY
DATA, COLLECTED FOR SPECIFIC
PURPOSE, ANY PURPOSE OR
WITHOUT PURPOSE CAN ALSO
YIELD INFORMATION FOR
HYPOTHESES FORMING.
SUMMARY
Descriptive epidemiology is hence
rightly called the hypothesis forming
stage of epidemiological sequence as
descriptive epidemiology is very
useful in providing immense
information regarding the various
variables like time, place, person,
clustering, etc to form the
hypothesis.