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D ESCRIPTIVE S TUDIES Hui Jin Department of epidemiology and health statistics School of Public Health

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1 D ESCRIPTIVE S TUDIES Hui Jin Department of epidemiology and health statistics School of Public Health


3 3 DESCRIPTIVE STUDIES Descriptive 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.

4 D ESCRIPTIVE E PIDEMIOLOGY 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?

5 D ESCRIPTIVE E PIDEMIOLOGY PERSON PLACE TIME Think of this as the standard dimensions used to track the occurrence of a disease. WHO WHEN WHERE WHAT? WHY??

6 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.

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

8 P ERSON WHO is getting the disease? Many variables are involved and studied, but factors such as sex, age & race often have a major effect.

9 C HARACTERISTICS OF P ERSON Age Sex Ethnic group Socioeconomic status Nativity Religion Marital status Occupation

10 A GE


12 S EX

13 T IME 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


15 C HARACTERISTICS R ELATING TO T IME Secular change (long-term) Point epidemics (short-term) Cyclic trends Seasonal variation


17 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

18 P OINT E PIDEMICS 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



21 C YCLIC T RENDS 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

22 Cyclic Trends

23 S EASONAL V ARIATION Seasonal variation can be seen for some diseases or conditions falling within a calendar year

24 S EASONAL V ARIATION Seasonal variation can be used to suggest possible etiology. Migratory Birds?

25 T IME C LUSTERING 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 Syndrome Staphylococcus aureus infection in women using newly introduced hyperabsorbent tampons

26 Time Clustering

27 P LACE WHERE are the rates higher? lower? Geographic location of source Geographic location of reservoir


29 5 C RITERIA OF P LACE  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)

30 C HARACTERISTICS R ELATING TO P LACE International Variation within countries Urban-rural Local Building Maps

31 P LACE Distribution of AIDS in the US 1990



34 I NTERACTIONS OF T IME AND P LACE Time-place clustering Migration

35 TYPES Ecological or Correlational studies Cross-sectional or Prevalence Studies Longitudinal or Incidence Studies

36 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.

37 ECOLOGICAL STUDIES Frequency and trends of the suspected causes/risk factors Prevalence or incidence of the disease occuring in a particular area Correlated with

38 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 hypothesis 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

39 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.

40 EXAMPLES OF ECOLOGICAL STUDIES Cancer cervix is rare in Jewish women due to male circumcision Sickle cell disease is more seen in Indian tribes

41 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.

42 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.

43 RAPID METHODS OF PREVALENCE STUDIES Cluster surveys Computer simulation Random digit dialing for sampling Computer 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.

44 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

45 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

46 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


48 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.

49 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).

50 DESCRIBING THE STUDY POPULATION Age Sex Occupations Socioeconomic status Literacy profile Social customs, habits Specific lifestyles Knowledge of health facilities available and their utilization

51 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.

52 DEFINING AND DESCRIBING THE DISEASE Defining the disease (case definition) An operational working definition to make uniform and unbiased counting in populations

53 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

54 JURISDICTION The dividing line between the descriptive and analytical studies is not very sharp.

55 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.

56 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

57 DATA AND HYPOTHESIS Keen observation of any data, collected for specific purpose, and purpose or without purpose can also yield information for hypotheses forming.

58 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.

59 Q UESTION ? Assigned readings, session 2: Grimes DA, Schultz KF. Descriptive studies: what they can and cannot do. Lancet 2002;359:145-9.

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