Types of Studies

Aim of epidemiological studies To determine distribution of disease To examine determinants of a disease To judge whether a given exposure causes or prevents disease

Epidemiologic Design Strategies Descriptive studies –Populations Correlated studies –Individuals E.g. case-series, case reports, cross-sectional surveys Analytical studies –Observational studies Case-control studies Cohort studies –Intervention studies Clinical trials

Descriptive (person, place, and time) –Hypothesis generating Analytic (causal) – Hypothesis testing

Descriptive Studies Correlational studies (may be called ecological studies) – Describes the disease in the interpopulation in relation to the factor of interest (compare disease frequency between different groups at the same time or the same population at different time).

–Strength Quick and inexpensive, can be used as first step to measure a disease-exposure relationship –Limitation Doesn ’ t link specific persons ’ exposure with specific outcome Can ’ t control for potential confounding factors

Descriptive Studies Case Reports and Case Series –Describes single patient or group of patients experience with similar diagnosis reflecting usually unusual feature of a disease. –Most common form of study published in medical journals –Strengths May lead to formulation of new hypotheses Important link between clinical medicine and epidemiology –Limitation Cannot be used to test hypotheses

Descriptive Studies Cross-Sectional (or prevalence) Survey –Exposure and disease status are simultaneously assessed in a population –Strength Provides information about the frequency and characteristics of a disease –Useful for public health –Can provide information concerning the prevalence of disease or other health outcome in special groups (e.g. occupations) –Limitation Can ’ t determine whether exposure preceded or occurred as a result of the disease

Analytic Studies Explicit comparison of exposure and disease Groups are assembled to determine whether risk is different for exposed and unexposed Appropriate comparison group Hypothesis testing Two types –Observational - natural course of events –Intervention - investigator allocates exposure and follows subjects

Observational Studies Case-Control –Persons with disease –Comparison group Cohort –Subjects classified on basis of exposure of a factor –Follow-up to determine presence of disease Prospective vs. retrospective

Case-Control studies Subjects are selected on the basis of whether they do (cases) or don ’ t (controls) have a particular disease under study, the groups are then compared with respect to proportion having a history of unexposure or characteristic interest.

CASE-CONTROL STUDIES (Cont.) Advantages –Uniquely suited to diseases with long incubation periods –More efficient in terms of time and money –Good for study of rare disease –Can look at multiple exposures for a single disease Disadvantages –Inefficient for evaluation of rare exposures –Cannot directly compute incidence rates of disease –Particularly prone to bias (selection and recall in particular)

Cohort Studies A cohort study is a study where a group of individuals are followed. The study population is defined on basis on the presence or absence of exposure to a suspected risk factor for a disease.

Cohort study is undertaken to support the existence of association between suspected cause and diseaseCohort study is undertaken to support the existence of association between suspected cause and disease A major limitation of cross-sectional surveys and case-control studies is difficulty in determining if exposure or risk factor preceded the disease or outcome. Cohort Study: Key Point: –Presence or absence of risk factor is determined before outcome occurs.

Cohort studies –longitudinal –Prospective studies –Forward looking study I –Incidence study starts with people free of disease assesses exposure at “ baseline ” assesses disease status at “ follow-up ”

Cohort studies Strengths We can find out incidence rate and risk More than one disease related to single exposure can establish cause - effect good when exposure is rare minimizes selection and information bias Weaknesses losses to follow-up often requires large sample ineffective for rare diseases long time to complete expensive Ethical issues

INTERVENTIONAL STUDY Interventional studies are when researchers attempt to show that A causes B by actually manipulating those factors they think have something to do with causing some outcome.

Intervention Studies - explore the association between interventions and outcomes. (Experimental studies or clinical trials)

Examples of Experimental Epidemiologic Studies Prophylactic vaccines tested on children populations to prove the efficacy of the vaccines in preventing the diseases (i.e., polio) Prophylaxis with drugs in preventing disease (i.e., penicillin to prevent rheumatic fever) Impact on health-related behavior and coronary heart disease in response to community-wide heart disease prevention intervention

Experimental Study Design Time Sample of Cases Treated (T) Not Treated (NT) (Control) Treated - Improved Treated – Not Improved Not Treated - Improved Not Treated – Not Improved

Interventional Study On lab animals Clinical Trials: 1.single-blinded 2. double-blinded

Advantages Helpful in assessing the value of new therapies to combat acute diseases in developing countries Prospective design Eliminates bias by comparing two otherwise identical groups Allows for meta-analysis

Disadvantages Expensive and time consuming Not always properly conducted – too few subjects, too short a time period