STUDY DESIGNS

STUDY DESIGNS Epidemiologic studies broadly classified as: Observational - Non-Interventional

Experimental - InterventionalOBSERVATIONAL:

Here nature allowed to take its course. Changes and differences in one variable are studied in relation to changes and differences in another. Investigator DOES NOT intervene. - (e.g. Smoking v/s lung cancer: Investigator does not decide who smokes and who must not).EXPERIMENTAL:Investigator deliberately introduces change in one variable and measures effect of this on outcome.-These have ethical problems in humans as well as costs involved. -Therefore most studies are OBSERVATIONAL.Observational studiesThese range from-Relatively weak studies .e.g Case reports and Ecological studiesto -Strong studies e.g. Cohort and Case-Control studies.

Observational studies cont..:Observational studies are either:-Descriptive -Analytical

Descriptive studies:Weakest epidemiological design.

Investigator merely describes health status of pop. or characteristics of a number of patients by time, place and person.

It offers limited information about group of patients, their clinical characteristics and outcomes.Descriptive Studies cont:These studies are weak because they make no attempt to link cause and effect thus no causal association can be determined.

However, are often first step to well designed epidemiological study.

Can define good hypothesis to be tested using a stronger design.

DESCRIPTIVE STUDIES contDescription by -Person - stating what pop. and sub pop. do or do not develop disease.-Place Its geographical location

-Time - How does frequency of disease vary with time.

DESCRIPTIVE STUDIES contIndices of person include demographic e.g.:Sex, age, life style e.g. alcohol use and medicines.

Characteristics of place e.g.:Variation between regions, or countries

Time characteristics e.g.: -Season variation, Frequency with time.

DESCRIPTIVE STUDIES cont:Examples of Descriptive studies:-Case reports and Case series.

Case reports -Considers single or grp of patients with similar diagnoses.

-Clinician identifies unusual feature of disease and formulates a hypothesis.

-They represent important interface between clinical medicine and epidemiology.DESCRIPTIVE STUDIES cont:Case reports and series are among most published in journals (Over 1/3 of articles).

Case series are collection of individual case reports which may occur within a fairly short time period.

Case-Series and Case Reports:This study type has important historical place:-Was used as early means to identify presence or beginning of epidemic.

-Even now, in routine surveillance, case reports can suggest new disease or epidemic.

12Case-Series and Case Reports:In investigating affected individuals they can lead to hypothesis formulation.

An analytic study, may then be conducted to compare the experiences of individuals with disease to identify possible causal factors

Case-Series and Case Reports cont:e.g. Early epidemiology of AIDS:Between October 1980 and May 1981, 5 cases of previously healthy homosexuals in U.S. had pneumo-cystis Carinii pneumonia (PCP)

PCP was known to occur in older cancer patients with immunosuppression due to chemotherapy.

Case-Series and Case Reports cont:1981 young homosexuals seen with Karposis Sarcoma.-Again this was known mostly common in elderly men and women equally.-Thus hypothesis was formulated.While case reports and case series are very useful for hypothesis formulation;-They cant be used to test valid statistical association.Case-Series and Case Reports cont:Limitations include fact that:-Based on experience of one person. Presence of any risk factors may be co-incidental.-Frequently not large enough to permit quantification of frequency of an exposure.-Interpretation limited for lack of appropriate control group.

OBSERVATIONAL STUDY: (analytical)CORRELATIONAL (ECOLOGICAL) STUDIESEcological (Correlational):Are also weak designs.-Units of study are populations other than individuals.-Useful also in generating hypotheses but no causal inference can be drawn. *An apparent ecological link may not be a true link.-May be confounded by several factors.Ecological (Correlational).:Investigator measures characteristic of entire population to describe disease in relation to age, sex etc.E.g. Pap Smears Correlation with mortality from cancer of the Cervix.-For example the % in Cancer Cervix in 2 periods.-1950-1954 - Time pap started to be used widely-1960-1964 - Time notable in mortality could have startedEcological (Correlational)..:% of women with pap screening was noted in different states.States with highest % of women screened had strong and significant correlation and had largest % in mortality.Conversely those with lowest % showed significant smallest % in mortality.

ECOLOGICAL (CORRELATIONAL):

Example raises question (hypothesis) that:Screening mortality from Ca. Cervix.Hypothesis cant be tested (answered) with this data since:a) Dont know whether women screened are those who experienced lower mortality i.e. The survey involved ALL women NOT only those screened.ECOLOGICAL (CORRELATIONAL):

b) Cant control for potential confounding factors.E.g. -In Study of per capita average daily intake of pork in relation to Breast Cancer. -A strong +ve Correl. between death from Ca. breast and pork eating was noted. (Hypothesis).-However pork eating may be marker to other factors e.g.-fat in body)- vegetable eating) - These risk of breast Cancer.- Socio-economic status)

Ecological (Correlational):Cant separate confounders using correlational data.-More dramatic illustration of this limitation is:-Very strong +ve correlation between per capita number of colour TV sets and CHD mortality in different nations.Hypothesis is there is association between colour TVs and CHD mortality.-However colour TVs are related with other lifestyles and these risk of CHD e.g. smoking, cholesterol, inactivity.Ecological (Correlational):-Again absence of correlation does not mean absence of valid statistical associations. E.g. In early 70s use of oral contraceptives in USA. At same time mortality rates from CHD among all childbearing women 30%.-This information does not support a +ve association between use of Oral Contraceptives and risk of fatal CHD.

ECOLOGICAL (CORRELATIONAL):Later, Cohort and Case-Control studies have shown a two fold in risk of fatal CHD in women using OCs than those who dont.This is difficult to get or perceive by correlational or ecological data.ECOLOGICAL (CORRELATIONAL):Correlation data provides average exposure levels rather than actual individual levels.*So an overall +ve or ve linear association may be shown but may be masking more complicated relations between exposure and disease.Ecological (Correlational).:E.g. In various countries the per capita alcohol consumption showed a striking simple inverse linear relationship with CHD mortality.This meant that:-Countries with per capita alcohol consumption had CHD mortality risk

Countries with per capita alcohol consumption had CHD mortality.

ECOLOGICAL (CORRELATIONAL):Later it was shown that the relationship is not this simple.Actually the association is best represented by a J-shaped curve.Those consuming largest amounts (C) have highest risk.Those drinking small to moderate amounts (B) have lowest risk.

ABCAlcohol ConsumptionCHD RateEcological (Correlational):Those who dont drink have slightly (A) higher risk than (B).Such non-linear relationship cannot be identified easily from correlation studies in which:Exposure represents an average consumption for population rather than the actual consumption patterns of individuals.

OBSERVATIONAL STUDIES (DESCRIPTIVE)CROSS-SECTIONAL STUDIESCROSS-SECTIONAL StudiesAlso Descriptive studies sometimes called prevalence surveys.-Here exposure and disease are assessed simultaneously in individuals of well defined pop. (Diarrhoea/Bacterial infection).Its like a specific window e.g. calendar year, in which community wide survey is conducted*OR:Cross-sectional studies.:Fixed point in the course of events that varies in real time from person to person e.g.Pre-employment examPre-school entrance examThese provide information about:-Frequency and Characteristic of disease e.g. affects men only by giving a snapshot of health experience of pop. at specific time. E.g. KDHS FP; morbidity etc.

CROSS-SECTIONAL Studies.:Cross-sectional studies also useful for disease prevalence etc in certain occupations.But since exposure and disease are assessed at a single point in time, one CANNOT determine whether exposure preceded or resulted from the disease.

Hypothetical SituationOCCUPATIONAL EXPOSURE & DISEASE PREVALENCE MEASURED BY X-SECTIONAL SURVEY:

Occupational exposure & Disease prevalence by X-sectional survey:JOB A has known hazardous exposure.JOB B doesnt-If conduct X-Sectional study at X, then more in A have respiratory disease than in B.Ratio A:B= 4.020:5-If conduct study at Y, 15 will be in (B) and 10 in (A).Thus prevalence will be:A = 10 and B=15 90 110 37dOccupational exposure & Disease prevalence by X-sectional survey:Ratio A:B = 0.8At Y greater prevalence in B not cause Job is more hazardous but cause 10 from A chose to change to less hazardous. -So being in B at point Y for those with respiratory symptom is:The effect of the symptoms, not the cause.This type of chicken/egg dilemma is common in virtually all X-sectional studies.

X-Sectional Studies Cont:Another example:-Investigators compared prevalence rates of CHD among white farmers between;a)Those who did their own labor.V/Sb)Those who did not do their own labor.39eeX Sectional studies..:Prevalence rates of CHD among (b) were X5 higher than those in (a) i.e. 157.2 versus 33.3/1000. (Even after adjusting for age).

-It is not possible to know whether it is truly protective or those with CHD are likely to reduce their physical labor.

X-Sectional studies cont.:X-sectional studies consider prevalence rather than incidence.-They reflect determinants of survival and aetiologyE.g.- A study shows low prevalence of CHD among blacks than whites.- Since there is nothing to show CHD develops less in blacks than whites.X-Sectional studies cont.:It could be :True that whites develop more CHD orBlacks have higher rates of CHD but die at higher rates.So X-sectional data can not distinguish between the two.

X-Sectional studies cont.:In most instances X-sectional data is used to describe characteristic of individuals with disease and formulate hypotheses, but not test them.Chart110320

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