Epidemiological Study Designs Lecture Notes PDF

Epidemiological Study Designs Epidemiological studies may be descriptive or analytical; - Descriptive studies look at the distribution of a disease or health problem in a community. - Analytical studies look at the determinants (i.e. factors enhance or inhibit) occurrence of diseases in populatio...

Epidemiological Study Designs Epidemiological studies may be descriptive or analytical; - Descriptive studies look at the distribution of a disease or health problem in a community. - Analytical studies look at the determinants (i.e. factors enhance or inhibit) occurrence of diseases in populations  Types of epidemiological study designs I- ObservationalStudies: A. Descriptive studies B. Analytic studies: Case report Case-control study Case series Cohort study Correlation study Cross-sectional study Cross-sectional study II- Experimental (interventional) Studies Preventive trials Clinical trials Population trials  A. Descriptive Studies Describe general characteristics of distribution in relation to time, place and person (the basic triad of descriptive epidemiology) Person include distribution by age, sex, race, and marital status, and occupation, life style variables as food, smoking and medicine … etc Place refer to geographical distribution of disease, urban, rural, countries variation. Time include secular trend, seasonal variation. It is the first clue for disease determinant in order to formulate hypothesis.  a- Case report: is a detailed report of the symptoms, signs, diagnosis, treatment, and follow-up of an individual patient. Case report may contain a demographic profile of the patient, but usually describe an unusual or novel occurrence.  b- Case Series: is a type of medical research study that tracks subjects with a known exposure, such as patients who have received a similar treatment, or examines their medical records for exposure and outcome.  Analytic Studies 1- Cross sectional study 2-Case control study 3-Cohort study  1- Cross-Sectional Study (Prevalence study) Study which give an answer to the current situation at a given point in time. Cross-sectional study is mainly a descriptive study but may be analytic if the relationship between disease and risk factor is done during analysis and distribution of disease in relation to time, place and person. Both exposure (risk factors) & outcome (diseases) are present. Therefore, we cannot determine if exposure preceded disease or not.  It measures prevalence (all cases), not incidence (= new cases) of disease Not suitable for studying rare or highly fatal diseases or a disease with short duration of expression (acute diseases)  Subjects and methods: The whole population if confined may be taken or representative sample if the target population is large. No controls are used in cross sectional study.  Examples: 1. Census 2. Survey study 3. Screening test Advantages: 1. Measure population sample characteristics 2. Determine disease prevalence. 3. Can study multiple risk factors and multiple diseases at the same time. Disadvantages: Show associations but do not indicate causal relationships.  2- Retrospective study (Case control study) In a retrospective study, persons who diagnosed as having a disease (cases) are compared with persons who do not have the disease (controls); with regard to their past exposure to the possible risk factor of interest. Advantages: 1- The number of subjects can be small. 2- Results can be obtained relatively quickly. 3-Low cost.  Disadvantages: 1. Selection bias: error in selection of cases and controls. 2. Information bias: unavailable or inaccurate records. 3. Recall bias:- inaccurate recall of events in the distant past 4. Confounding bias:- there are variables that increase or decrease the association between the disease and the factor under study. Uses: Particularly useful for etiologic study of rare diseases (high exposure)  3- Prospective study (Cohort study) A cohort is a group of persons who share a common attribute (feature or experience) e.g. a birth cohort are person born in the same year in same period of years. A cohort study starts with 1 groups of people (cohort) free of the disease under study. They choose to be exposed to the risk factor or not. The cohort is followed over time to determine differences in the rates of disease development (or rate of death from disease).  Types of cohort study:- a. Concurrent prospective cohort study. b. Non- Concurrent prospective cohort study or historical study or retrospective cohort study: in this study, exposure has been started in the past and documented in records. This type of design is used to reduce the duration of the study. Exposure is detected in the past and measurement of development or non-development of disease is ascertained at the time when study begins or followed longitudinally from time of start of research for more time in the future.  Advantages: 1. Lack of bias in ascertaining the exposure, since cohort is classified as to the state of exposure before the disease develops. 2. Permits calculation of incidence rates, thus both relative risk and attributable risk can be calculated. 3. Permit observation of development of additional diseases as by product e.g effects of Chernobyl nuclear explosion.  Disadvantages: 1. Require large number of subjects. 2. Long follow up period. 3. Attrition: - (loss of subjects from follow up) because of disinterest, migration, or death from other causes or potential loss of staff and/or funding. 4. Potential change of exposure status of subjects over time. 5. Changes in diagnostic criteria and methods over time with advances in technology. 6. Very costly. Uses: Particularly useful to study outcome when exposure is rare, but incidence among exposed is high.  Interventional studies 1-Clinical trial 2-Quasi experimental study Clinical trial Prospective study 1. Randomization:- The 1. Subjects select investigator randomly themselves for determines who will be exposure or non- exposed (treated) or not exposure to the factor 2. Blinding is applied 2. No blinding is applied Blinding may be: 1. Subject under investigation............. single blind 2. Subject and data collector...............double blind 3. Subject + investigator+ data analysts.....triple blind Blindness can be achieved by using a similar shape or color and taste…or using placebo. Problems encountered include: 1) Ethical issues 2) Non participation, attrition or non-compliance. 3) Correctly defining inclusion and exclusion criteria. 4) Sample size. 5) Expenses. 6) Long period of follow up Phase 4 Phase 3 Phase 2 Phase 1 Several From 100 to Less than 15-30 Number of hundreds or - To further thousands - To 100 - To - To find safe participants Purpose thousands evaluate the compare the determine if dosage long-term new agent the agent - To decide safety and with the has an how the effectivenes current effect on agent s of a new standard particular should be treatment disease given - To see how - To observe the agent how the affects the agent human body affects the human body Thank You