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Descriptive Studies Hui Jin

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1 Descriptive Studies Hui Jin
Department of epidemiology and health statistics School of Public Health

2 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 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 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?

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

6 Generally, stringent criteria for case definitions are desirable.
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 WHO is getting the disease?
Person WHO is getting the disease? Many variables are involved and studied, but factors such as sex, age & race often have a major effect.

9 Characteristics of Person
Age Sex Ethnic group Socioeconomic status Nativity Religion Marital status Occupation

10 Age

11 Age

12 Sex

13 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


15 Characteristics Relating to Time
Secular change (long-term) Point epidemics (short-term) Cyclic trends Seasonal variation

16 Secular Change

17 Incidence Rates of Cancers in Women Incidence Rates of Cancers in Men
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

18 Short-term changes occur over limited time frames
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

19 Point Epidemics

20 Point Epidemics

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

24 Seasonal variation can be used to suggest possible etiology.
Migratory Birds?

25 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 Syndrome Staphylococcus aureus infection in women using newly introduced hyperabsorbent tampons tampons卫生棉条

26 Time Clustering

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

28 John Snow and Cholera

29 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)

30 Characteristics Relating to Place
International Variation within countries Urban-rural Local Building Maps

31 Distribution of AIDS in the US 1990
Place Distribution of AIDS in the US 1990

32 Local

33 Building Maps

34 Interactions of Time and Place
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. These are the usual initial epidemiological studies. These are usually descriptive and can also be analytical when applied to individuals in a specific group.

37 Frequency and trends of the suspected causes/risk factors
ECOLOGICAL STUDIES Frequency and trends of the suspected causes/risk factors Prevalence or incidence of the disease occuring in a particular area Correlated with These are the usual initial epidemiological studies. These are usually descriptive and can also be analytical when applied to individuals in a specific group.

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 DISADVANTAGES cannot be not done at individual level hypotheses generated are subject to ecological fallacy ( see next slide)

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.

Cancer cervix is rare in Jewish women due to male circumcision Sickle cell disease is more seen in Indian tribes 镰状细胞病

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. Conducted to know about all the cases present (all the existing cases, old and new cases, all current cases) in that community i.e. Prevalence of the disease..

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. It is a one time study, but can be extended to include historical information that can be easily collected at the same time, in which house to house information was obtained about all cases present. Cross sectional studies can be conducted in community viz. on leaders, mothers, health workers, using interviews, group discussions, direct observations, and household interviews.

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 Time consuming and expensive
Subjected to recall bias and confounding bias Other common biases encountered in these studies are of berkesonian, information and investigator types Under-represent people with a short-course of disease Limited to studies of causes that are of long standing nature The validity of the associations derived of these studies depends on careful elimination of the artificial associations and confounding effects.

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 OTHER NAMES :- INCIDENCE STUDY ( AS INCIDENCE CAN BE MEASURED DIRECTLY), FOLLOW UP STUDY (AS CONTINUOUS FOLLOW UP IS ESSENTIAL) PROSPECTIVE STUDY (AS THE DIRECTION OF THE STUDY IS FORWARD-LOOKING. Cohort studies are of this design. These studies, though yield incidence directly, are time consuming and expensive when compared to cross- sectional onetime studies. It should be remembered that the follow up given during the disease management e.g. follow up of patients with tuberculosis during treatment, is not to be confused with these longitudinal studies.

46 Defining and describing the disease (Operational case definitions
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

47 POPULATION AT RISK It should be remembered that entire population is not going to be studied except the relevant population . For e.g. for cancer cervix studies, we choose the women in reproductive age, not female children and certainly not males. Reference population is the one, which has to be get benefited after our descriptive study. Thus the population at risk is better one to be studied rather than the total population while formulating hypothesis as the inferences after testing hypothesis cannot be generalized to all the components of whole population except to the specific and vulnerable/susceptible portion of it.

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. For example, if we are studying Scabies incidence in a sample of school children, all the school children comprise our reference population. It is the population to which ,we apply or extrapolate the end results or inferences obtained after testing our hypothesis. These populations may be as narrow and small as exposed groups or as big and universal as for example, all reproductive women. It usually serves as denominator to calculate rates and ratios to quantify the disease/exposure load

49 A representative portion of it (sample).
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). This serves as a refined denominator for calculating and quantifying the disease load. The study population characteristics have to be described in detail for proper formulation and elucidation of hypothesis.

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

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 the disease (case definition) An operational working definition to make uniform and unbiased counting in populations

53 APPLICATIONS To formulate causal hypothesis.
Indicate the disease load and frequency alterations and thereby help to make future projections. Diagnosing and telling the prognosis Disease clustering offers the good opportunity to study the natural history of disease and also for controlling it e.g. Observation by John Snow of cholera clustering at Broad street in London square led to understanding of mode of transmission and its control. Disease mapping is an useful tool to know the geographical pathology and for health planning basing on local conditions and resources

54 JURISDICTION The dividing line between the descriptive and analytical studies is not very sharp. as individuals, groups , communities and the total population and their characteristics are interrelated and intermingled and any study conducted on them is flexible and not typical. Ecological study can be a group based analytical study in addition to be of descriptive nature. Cross-sectional study can be both descriptive as well as analytical one, descriptive when one variable or each variable in a group or population is studied, analytical as it provides information about the presence and strength of association. (oxford textbook of public health)

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. The line between the ecological and individual contexts is the dividing line between descriptive and analytical studies and is not sharp and often overlapping.e.g cross sectional studies can be both descriptive and analytical. One practical distinction between the descriptive and analytical studies is that former are based on the study and analysis of routinely collected data while the later from ad hoc collection of information.

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 Several hypotheses can be made suspecting the time, place, person variables obtained from descriptive studies as causes for the disease occurrence or its altered frequency. This is the very usual way of getting ideas about hypothesis formation. If one thinks and analyses why a particular disease has occurred at a particular time or place and why it has affected people of certain ages and of certain groups only, then a number of hypotheses will emerge. Ecological studies, the studies conducted on specific groups with specific characteristics, also provide useful information for forming the hypothesis. Examples are that Cancer cervix is rare in Jewish women due to male circumcision and Sickle cell disease is more seen in Indian tribes.

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. Political & Natural observation of Bills of Mortality in London “ by John Graunt yielded lots of hypotheses to be verified. It is often observed in the development of science that many unexpected discoveries were made while observing for some thing else. Data treatment in terms of tables , graphs, rates and ratios will provide material for hypothesis formulation.

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. 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 Question? 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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