DESCRIPTIVE STUDIES

Hui JinDepartment of epidemiology and health statisticsSchool of Public Healthjinhuimc@163.com

A DESCRIPTIVE STUDY A DESCRIPTIVE STUDY IS “CONCERNED WITH AND DESIGNED ONLY TO DESCRIBE THE EXISTING DISTRIBUTION OF VARIABLES,WITHOUT REGARD TO CAUSAL OR OTHER HYPOTHESES.”

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DESCRIPTIVE STUDIESDescriptive studies often represent the first

scientific toe in the water in new areas of inquiry.

Case reports, case-series reports, cross-sectional studies, and surveillance studies deal with individuals, whereas ecological correlational studies examine populations.

A frequent error in reports of descriptive studies is overstepping the data: studies without a comparison group allow no inferences to be drawn about associations, causal or otherwise.

DESCRIPTIVE EPIDEMIOLOGY Traditional descriptive epidemiology has focused on

three key features: person, place, and time,4 or agent, host, and environment.5

An alternative approach is that of newspaper coverage. Good descriptive research, like good newspaper reporting, should answer five basic “W” questions—who, what, why, when, and where—and an implicit sixth question, so what?

DESCRIPTIVE EPIDEMIOLOGY

PERSON

PLACE

TIME

Think of this as the standard dimensions used to track the occurrence of a disease.

WHO

WHEN

WHERE

WHAT?

WHY??

WHAT-CASE DEFINITION

Development of a clear, specific, and measurable case definition is an essential step in descriptive epidemiology.

Generally, stringent criteria for case definitions are desirable.

WHY

Why did the condition or disease arise? Descriptive studies often provide clues about cause that can be pursued with more sophisticated research designs.

PERSON

WHO is getting the disease?Many variables are involved and studied,

but factors such as sex, age & race often have a major effect.

CHARACTERISTICS OF PERSON

Age Sex Ethnic group Socioeconomic status Nativity Religion Marital status Occupation

AGE

AGE

SEX

TIME WHEN does the disease occur?

“Temporal” Range from hours to decades

Type of disease dictates “time” element to be used Graphic format often used

y-axis (vertical) - frequency x-axis (horizontal) - time

CHARACTERISTICS RELATING TO TIME

Secular change (long-term) Point epidemics (short-term) Cyclic trends Seasonal variation

SECULAR CHANGE

SECULAR CHANGE

• Secular changes (“temporal variation”) occur slowly over long periods of time Longer than one year

Incidence Rates of Cancers in Women

Incidence Rates of Cancers in Men

POINT EPIDEMICS• Short-term changes occur over limited

time frames Hours Days Weeks Months

• Used for short-term exposures or diseases with short incubation and/or illness durations

POINT EPIDEMICS

POINT EPIDEMICS

CYCLIC TRENDS• Cyclic trends may be either long-term or

short term events.

• Some are “seasonal” while others are cyclic due to other factors: Immigration School year Military deployment

Cyclic Trends

SEASONAL VARIATION

• Seasonal variation can be seen for some diseases or conditions falling within a calendar year

SEASONAL VARIATION

• Seasonal variation can be used to suggest possible etiology.

Migratory Birds?

TIME CLUSTERING

• Time clustering data can sometimes be used to trace the “beginning” to the introduction of a specific causal agent Thalidomide & birth defects

First marketed in Europe in 1950’s as sleeping pill and to treat morning sickness in pregnant women

Toxic Shock SyndromeStaphylococcus aureus infection in women using newly introduced hyperabsorbent tampons

Time Clustering

PLACE

WHERE are the rates higher? lower?Geographic location of sourceGeographic location of reservoir

JOHN SNOW AND CHOLERA

5 CRITERIA OF PLACE

Rate observed in all ethnic groups in the area Rate NOT observed in persons of similar

groups inhabiting other areas Healthy persons entering area get ill at same

frequency People who leave do NOT show similar levels Similar levels of infestation in other species (if

zoonotic disease)

CHARACTERISTICS RELATING TO PLACE

International Variation within countries

Urban-rural Local

Building Maps

PLACE

Distribution of AIDS in the US 1990

LOCAL

BUILDING MAPS

INTERACTIONS OF TIME AND PLACE

Time-place clusteringMigration

TYPES

Ecological or Correlational studies

Cross-sectional or Prevalence Studies

Longitudinal or Incidence Studies

ECOLOGICAL STUDIES

Studies conducted in specific population having specific characteristics in a specified geographical area

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 STUDIES

Frequency and trends of the suspected causes/risk factors

Prevalence or incidence of the disease occuring in a particular area

Correlated with

ADVANTAGES AND DISADVANTAGES

Advantages: Conducted at group level, not at individual level,

hence relatively easy to do and quickUse existing dataGenerate and support new hypothesisEcological 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)

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.

They are conducted for chronic diseases having high prevalence with very low incidence.

PREVALENCE STUDY METHODOLOGY

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).

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. They seek the information of about the individuals in a group or population.

RAPID METHODS OF PREVALENCE STUDIES

Cluster surveysComputer simulationRandom digit dialing for samplingComputer based interviews & use of spreadsheet

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 bias3. 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.

INCIDENCE STUDY, FOLLOW UP STUDY,

PROSPECTIVE STUDY

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 Sex 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

APPLICATIONS1. 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, and 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.

QUESTION?

Assigned readings, session 2:

Grimes DA, Schultz KF. Descriptive

studies: what they can and cannot do.

Lancet 2002;359:145-9.