Epidemiology Study Design PDF

METHODOOGY EPIDEMIOLOGICAL STUDY DESIGN Dr. Sijal Fadhil Farhood AL-joborae F.I.C.M.S (Baghdad) M.Sc. Community (Nahrain) M.B.Ch.B (Babylon University) INTRODUCTION The primary concern of epidemiologist is to study disease occurrence in people ,who during the course of their lives are exp...

METHODOOGY EPIDEMIOLOGICAL STUDY DESIGN Dr. Sijal Fadhil Farhood AL-joborae F.I.C.M.S (Baghdad) M.Sc. Community (Nahrain) M.B.Ch.B (Babylon University) INTRODUCTION The primary concern of epidemiologist is to study disease occurrence in people ,who during the course of their lives are exposed to numerous factors and circumstances, some of which may have a role in disease aetiology. Unlike the clinician or the laboratory investigator, who is able to study the disease condition more precisely ,the epidemiologists employs carefully designed research strategies to explore disease aetiology.  CLASSIFICATION OF EPIDEMIOLOGICAL STUDIES 1- descriptive studies. A-case reports and case series. B-correlation studies. C-cross sectional studies. 2-Analytic studies: A-observational studies: 1-case control study. 2-cohort study. B-interventional (experimental)studies: DESCRIPTIVE EPIDEMIOLOGY Describes the pattern of disease occurrence in terms of person, place, time model or host , agent ,environment model. Defines the relationships of disease to the population at risk. ADVANTAGES OF DESCRIPTIVE STUDIES 1-They use available data , so there is less time,less efforts and money. 2-Describe disease patterns , so importance include -For the epidemiologists : They can identify risk factors and formulate research questions. -For the health administrators: identify population at risk (whom & where) to allocate resources and preventive programs. Correlation or Ecological Studies They are based on aggregate measures of exposure and outcome from several populations. The population is the unit of observation available for study. Advantages 1-Quick & easy as it employs available data. 2-It is the first step in investigating a possible exposure diseased relationship. Disadvantages 1-It cannot link exposure to disease in individuals (unknown temporality) but gives average exposure. 2-It cannot control confounders. Case report & case series report: case report : The individual is the unit of observation available for study. Clinical case with “unusual” clinical picture They describe the experience of a single patient or a small number of patients with a similar diagnosis reflecting unusual features of a disease. They help in: -Formulation of a hypothesis suggesting an etiological association -Represent the first clues in identification of new disease or epidemic. Case series First case report may stimulate compilation of additional case reports….a case series Advantages: 1-Use available clinical data 2-Detailed individual data 3-Suggests need for investigation (hypothesis generation) Disadvantages: 1-May reflect experience of one person or one clinician 2-No explicit comparison group Cross sectional (prevalence) study 1-Collection of data on several individuals at “one point” in time. 2-Determines prevalence at a point in time 3-Therefore, CS is a prevalence study 4-The exposure and disease status are assessed simultaneously among individuals in a well defined population. 5-Snapshot in time  Example We are interested in the possible relationship of increased serum cholesterol level (the exposure) to electrocardiographic (ECG) evidence of CHD (the disease). We survey a population; for each participant we determine the serum cholesterol level and perform an ECG for evidence of CHD. This type of study design is called a cross-sectional study because both exposure and disease outcome are determined simultaneously for each subject; it is as if we were viewing a snapshot of the population at a certain point in time.  Advantages of cross sectional study Provides information on frequency and characteristics of the disease Determines prevalence of disease or any health phenomenon. Standardized data collection tool. Able to focus data collection in specific locations or specific groups of persons. May make comparisons among study participants. Relatively quick to do. May be repeated to get data on trends.  Limitations Inability to determine the temporal relationship between exposure and disease. May be biased by lack of participation Reflects prevalent, not incident cases DESIGN  Disease No disease exposed a b Not exposed C d No No Disease disease Disease disease exposed a b exposed a b Not exposed c d Not exposed c d To determine weather there is an association between exposure and disease we have two choices: 1-We can calculate the prevalence of disease in persons with the exposure( a/a+b),and compare it with the prevalence of disease in person without the exposure(c/c+d). 2-or we can compare the prevalence of exposure in persons with the disease( a/a+c),to the prevalence of exposure in persons without the disease(b/b+d)