Upload
claribel-morton
View
213
Download
0
Embed Size (px)
Citation preview
DESCRIPTIVE STUDIES
Hui JinDepartment of epidemiology and health statisticsSchool of Public [email protected]
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.”
3
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
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 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
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
SEASONAL VARIATION
• Seasonal variation can be seen for some diseases or conditions falling within a calendar year
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
PLACE
WHERE are the rates higher? lower?Geographic location of sourceGeographic location of reservoir
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
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
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
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.