Presentation on theme: "Introduction to Epidemiology"— Presentation transcript:
1Introduction to Epidemiology and Study DesignsAdapted from original PPT by Thomas Songer, Ph.D. – University of Pittsburgh and the Supercourse Team.Adapted from original PPT by Thomas Songer, Ph.D. – University of Pittsburgh and the Supercourse Team.
2What is Epidemiology?Epidemiology may be regarded simply as the study of disease and health in human populations.Here disease may be any adverse health outcome such as pre-term birth, it may not necessarily be a disease such as cancer.Epidemiology is basically common sense that is systemized to examine the etiology and potential prevention factors in disease.
3Goals of Epidemiological Research Describe the health status of populations by enumerating the occurrence of diseases, obtaining relative frequencies within groups and discovering important trends.Explain the etiology of diseases by determining factors that “cause” specific diseases or trends.Taken from the text “Epidemiological Research – Principles and Quantitative Methods” by Kleinbaum, Kupper, and Morgenstern.
4Goals of Epidemiological Research Predict the number of disease occurrences and the distribution of health status within populations.Control the distributions of disease in the population by prevention of new occurrences, eradication of existing cases, prolongation of life for those with disease, or otherwise improving the health status of afflicted persons.Taken from the text “Epidemiological Research – Principles and Quantitative Methods” by Kleinbaum, Kupper, and Morgenstern.
5Components of Epidemiology Measure disease frequencyQuantify diseaseAssess distribution of diseaseWho is getting disease?Where is disease occurring?When is disease occurring?Formulation of hypotheses concerning causal and preventive factorsIdentify determinants of diseaseHypotheses are tested using epidemiologic studiesOne of the standardized approaches is to evaluate the distribution of diseases, where does disease occur, and what are the patterns over time
6Types of Primary Studies Descriptive Studiesdescribe occurrence of an outcomeAnalytic Studiesdescribe the potential association between exposure and outcomeDescriptive studies examine the frequency to which diseases occur. Analytic studies evaluation the relationship of disease to different exposures
7Basic Question in Analytic Epidemiology Are exposure and disease linked?EDBasic designs in epidemiology examine if exposures are correlated with disease.ExposureDisease
8Basic Questions in Analytic Epidemiology Look to link exposure and diseaseWhat is the exposure?Who are the exposed?What are the potential health effects?What approach will you take to study the relationship between exposure and effect?In order to examine the link of exposure to disease, there needs to be standardized evaluation of exposure, as well as disease
9Basic Study Designs and their Application to Epidemiology • Society will be healthier• Society can save money on health care budgets• It will improve life expectancy• It will improve the economyResponses to this question vary. As outlined below, some believe that altruistically, society should pursue disease prevention to improve health and save money. These are noble goals, but I will demonstrate later that prevention does not always save money. The desire to prevent NCDs probably is a repercussion of our efforts to prevent infectious diseases. We have had great success in preventing communicable diseases. This has led to an initiative to prevent NCDs.
10Big Picture To prevent and control disease In a coordinated plan, look toidentify hypotheses on what is related to disease and may be “causing” itformally test these hypothesesStudy designs direct how the investigation is conductedPrevention and control, can be defined as the reduction of the incidence of disease .
11What designs exist to identify and investigate factors in disease?
12Study Designs Descriptive Analytic Case report Cohort study RCT Case seriesCase-ControlstudyStudy DesignsDescriptiveEpidemiologyCase-CrossoverstudyCross-sectionalEpidemiologic studies may be descriptive in nature (describing the frequency or characteristics of events) or analytic (testing relationships between common traits and outcomes). Differing forms of descriptive studies exist. These designs are outlined in the next slide.Analytic studies include experimental designs (the randomized controlled trial) and observational designs (case-control studies, cohort studies, etc.). The case-crossover study design has received a lot of attention in the past few years.studyBefore-AfterstudyEcologic study
13Timeframe of StudiesProspective Study - looks forward, looks to the future, examines future events, follows a condition, concern or disease into the future(looking forward)With a prospective study one starts with cohorts of well individuals, and we wait until events occurs.timeStudy begins here
14Timeframe of StudiesRetrospective Study - “to look back”, looks back in time to study events that have already occurred(looking back)Looking backward is often difficult because of recall bias,, however, the case control studies are very inexpensive in comparison with prospective studies.timeStudy begins heree.g. Case-Control Study
15Study Design Sequence Hypothesis formation Case reports Case series DescriptiveepidemiologyAnalyticepidemiologyAnimalstudyLabstudyClinicaltrialsThere are many different types of designs that are in the arsenal of the epidemiologists to investigate diseaseHypothesis testingCohortCase-controlCross-sectional
16Increasing Knowledge of DevelophypothesisDescriptive StudiesInvestigate it’srelationship tooutcomesCase-control StudiesIncreasing Knowledge ofDisease/ExposureDefine it’s meaningwith exposuresTo further illustrate, if one seeks to identify the etiologic factors (e.g. causal factors) behind an outcome (e.g. an MI), then each step in the epidemiologic framework provides new and important information.Descriptive studies are useful for identifying hypotheses to test in analytic studies. Case-control studies are then usually applied to evaluate if the hypothesized factor is related to the outcome of interest. Subsequently, cohort or longitudinal studies are applied to further define the importance of exposure to the causal agent for the development of the outcome.Cohort StudiesTest linkexperimentallyClinical trials
17Descriptive Studies • Society will be healthier • Society can save money on health care budgets• It will improve life expectancy• It will improve the economyResponses to this question vary. As outlined below, some believe that altruistically, society should pursue disease prevention to improve health and save money. These are noble goals, but I will demonstrate later that prevention does not always save money. The desire to prevent NCDs probably is a repercussion of our efforts to prevent infectious diseases. We have had great success in preventing communicable diseases. This has led to an initiative to prevent NCDs.
18Case ReportsDetailed presentation of a single case or handful of casesGenerally report a new or unique findinge.g. previous undescribed diseasee.g. unexpected link between diseasese.g. unexpected new therapeutic effecte.g. adverse eventsCase reports are in many ways “sentinel events” which can lead to testable hypotheses
19Case Series Experience of a group of patients with a similar diagnosis Assesses prevalent diseaseCases may be identified from a single or multiple sourcesGenerally report on new/unique conditionMay be only realistic design for rare disordersCase series also provide suggestive evidence many times leading to more extensive testing. Prevalence is how common the “disease” is at a specific point in time. Incidence is the rate of occurrence of the “disease”. For example, how many new cases do we see per year. Prevalence says what proportion/percentage of the population have the “disease” at a given point of time.
20Case Series Advantages Disadvantages Useful for hypothesis generation From Wikipedia, the free encyclopediaAdvantagesUseful for hypothesis generationInformative for very rare disease with few established risk factorsCharacterizes averages for disorderDisadvantagesCannot study cause and effect relationshipsCannot assess disease frequencyCase seriesFrom Wikipedia, the free encyclopedia
21cases with denominator Descriptive StudiesOne case of unusualfindingsCase ReportMultiple cases offindingsCase SeriesDescriptiveEpidemiology StudyDescriptive study designs include case reports, case series, incidence studies, and ecologic studies. The case report is the most elementary study design in the literature. It generally describes an injury or injuries to one or two individuals that have been identified in a medical setting. There is also usually a unique feature to the noted chronic disease . The case series design is an extension of the case report. In a case series, a number of events are described. These events usually have been observed over a set period of time (such as one year) and are identified from one reporting source (e.g. a hospital).The descriptive epidemiology study is noted by the collection of events over a defined population base and by the use of denominator data to determine rates. The most frequent information generated from these designs are incidence rates for injuries. The ecologic study is a hypothesis generating study. Usually using group-level data, it examines if two factors are correlated with each other.Population-basedcases with denominator
22Analytical StudiesResponses to this question vary. As outlined below, some believe that altruistically, society should pursue disease prevention to improve health and save money. These are noble goals, but I will demonstrate later that prevention does not always save money. The desire to prevent NCDs probably is a repercussion of our efforts to prevent infectious diseases. We have had great success in preventing communicable diseases. This has led to an initiative to prevent NCDs.
23Study Designs - Analytic Epidemiology Experimental StudiesRandomized Controlled Clinical Trials (RCT)Community trialsObservational StudiesGroup data (i.e. we don’t have subject level info)EcologicIndividual dataCross-sectionalCohortCase-controlCase-crossoverAn Introduction to Epidemiology (CDC)An Introduction to Epidemiology (CDC)
24Experimental StudiesTreatment and/or exposures occur in a “controlled” environmentPlanned research designsClinical trials are the most well known experimental design. Clinical trials use randomly assigned data.Community trials use nonrandom dataExperimental studies are stronger in determining the etiology of disease than descriptive studies
25Observational Studies Non-experimentalObservational because there is no individual interventionTreatment and/or exposures occur in a “non-controlled” environmentIndividuals can be observed prospectively, retrospectively, or currently (i.e. cross-sectional)
26Cross-sectional studies An “observational” design that surveys exposures and disease status at a single point in time (a cross-section of the population)timeCross sectional studies are some of the first studies completed because of ease and low costStudy only exists at this point in time
28Cross-sectional Design factor presentNo Diseasefactor absentStudypopulationfactor presentDiseasefactor absentCross-sectional studies examine a point in timetimeStudy only exists at this point in time
29Cross-sectional Studies Often used to study conditions that are relatively frequent with long duration of expression (nonfatal, chronic conditions)It measures prevalence, not incidence of diseaseExample: community surveysNot suitable for studying rare or highly fatal diseases or a disease with short duration of expressionCross-sectional studies involve point prevalence, not incidence. For very infrequent diseases they are of limited utility
30Cross-sectional studies DisadvantagesWeakest observational design, (it measures prevalence, not incidence of disease). Prevalent cases are survivorsThe temporal sequence of exposure and effect may be difficult or impossible to determineUsually don’t know when disease occurredRare events a problem. Quickly emerging diseases are also problem.Cross-sectional studyFrom Wikipedia, the free encyclopedia
31Epidemiologic Study Designs Case-Control Studiesan “observational” design comparing exposures in disease cases vs. healthy controls from same populationexposure data collected retrospectivelymost feasible design where disease outcomes are rareCase-control studies in epidemiology are the most used type of study design
32Case-Control Studies Cases: Disease Controls: No Disease Case-Control studies represent one form of analytic study that provides information on the relationship between causal factors and injuries. In a case-control study, subjects who have been injured are identified and their past exposure to suspected causal factors is compared with that of controls (persons who have not been injured).Many case-control studies ascertain exposure from personal recall, using either a self administered questionnaire or an interview. The validity of such information will depend in part on the subject matter. People may be able to remember recent events quite well. On the other hand, long term recall is generally less reliable.Source: Chapter 8: Case-control and cross-sectional studies, Epidemiology for the Uninitiated
33Case-Control Design Study begins here factor present Cases (disease) factor absentStudypopulationfactor presentControls(no disease)factor absentCase-Control DesignpresentpasttimeStudy begins here
34Case-Control Study Strengths Limitations Less expensive and time consumingEfficient for studying rare diseasesLimitationsInappropriate when disease outcome for a specific exposure is not known at start of studyExposure measurements taken after disease occurrenceDisease status can influence selection of subjectsCase control studies provide low cost answers to health questions.
35Hypothesis Testing: Case-Crossover Studies Study of event triggers within an individualStill a Case and Control component, but information on both components will come from the same individual who is ultimately a case.Case component = hazard period which is the time period right before the disease or event onset. For example, period of heavy lifting right before an MI.Control component = control period which is a specified time interval other than the hazard period. For example, presence or absence of a period of heavy lifting prior to time right before MI.Case cross over studies are the newest form of epidemiologic design.
36Epidemiologic Study Designs Cohort Studiesan “observational” design comparing individuals with a known risk factor or exposure with others without the risk factor or exposurelooking for a difference in the risk (incidence) of a disease over timebest observational designdata usually collected prospectivelyThe cohort studies is the best for observational studies as the environmental event can be assessed before any disease outcome
37Cohort Design Study begins here disease Factor present no disease populationfree ofdiseasediseaseFactorabsentno diseaseCohort DesignpresentfutureA cohort studies follows a cohort of individuals who do not have disease, and then identified over time those individuals who have an outcometimeStudy begins here
38Timeframe of StudiesProspective Study - looks forward, looks to the future, examines future events, follows a condition, concern or disease into the futureProspective cohort studyFrom Wikipedia, the free encyclopedia
39Prospective Cohort study ExposedOutcomeMeasure exposureand confoundervariablesNon-exposedOutcomeBaselineCase-control studies are perhaps the most frequent form of analytic study design. These designs are very good for events that are rare in occurrence..Still, there are some situations where cohort study designs would be appropriate in the field. The classic design in a cohort study is shown here. The study begins by assessing baseline levels of the exposure and other variables. Study subjects are then followed on a regular basis to identify the outcome. The frequency of outcomes are tested between persons who had exposure to the possible risk factor at baseline and persons with no exposure.timeStudy begins here
40Timeframe of StudiesRetrospective Study - “to look back”, looks back in time to study events that have already occurredtimeStudy begins hereProspective vs. retrospective studies
41Retrospective Cohort study ExposedOutcomeMeasure exposureand confoundervariablesNon-exposedOutcomeBaselineAn alternative form of the cohort study is something termed the retrospective cohort study. Other researchers may also call this a historical prospective study. This design is nearly identical to the prospective cohort study. The sequence of baseline exposure determination and longitudinal follow-up for outcomes is similar. The difference lies in the time in which the study begins. In this retrospective design, the researcher constructs the cohort study by looking back in time and placing data in the appropriate order and sequence. These studies are possible to do with large medical databases, such as the membership files of the Health Maintenance Organizations, or the medical files in the Scandinavian countries.timeStudy begins here
42Cohort Study Strengths Limitations Exposure status determined before disease detectionSubjects selected before disease detectionCan study several outcomes for each exposureLimitationsExpensive and time-consumingInefficient for rare diseases or diseases with long latencyLoss to follow-up-- Exp. Measured before disease - so no temporal ambiguity-- Exposure measured before disease - so disease cannot influence the amount of error with which exposure status is measured-- Subject selection before disease, disease status does not influence of subjects
43Experimental Studies Investigator can “control” the exposure Akin to laboratory experiments except living populations are the subjectsGenerally involves random assignment to groupsClinical trials are the most well known experimental designThe ultimate step in testing causal hypothesesExperimental studies are the ultimate form of design in assessing causality as there is random assignment to groups.
44Experimental StudiesIn an experiment, we are interested in the consequences of some treatment on some outcome.The subjects in the study who actually receive the treatment of interest are called the treatment group.The subjects in the study who receive no treatment or a different treatment are called the comparison group or control group.Experimental Studies
45Epidemiologic Study Designs Randomized Controlled Trials (RCTs)a design with subjects randomly assigned to “treatment” and “comparison” groupsprovides most convincing evidence of relationship between exposure and effectnot possible to use RCTs to test effects of exposures that are expected to be harmful, for ethical reasonsRCT should be conducted for hypotheses that have not been tested. Great care must be taken to evaluate possible negative outcomes as well as positive outcomes
46Experimental Design time Study begins here (baseline point) outcome RANDOMIZATIONInterventionno outcomeStudypopulationoutcomeControlno outcomeExperimental DesignbaselinefutureExperimental and observational studiesA common goal for a statistical research project is to investigate causality, and in particular to draw a conclusion on the effect of changes in the values of predictors or independent variables on dependent variables or response. There are two major types of causal statistical studies: experimental studies and observational studies. In both types of studies, the effect of differences of an independent variable (or variables) on the behavior of the dependent variable are observed. The difference between the two types lies in how the study is actually conducted. Each can be very effective. An experimental study involves taking measurements of the system under study, manipulating the system, and then taking additional measurements using the same procedure to determine if the manipulation has modified the values of the measurements. In contrast, an observational study does not involve experimental manipulation. Instead, data are gathered and correlations between predictors and response are investigated.From Wikipedia, the free encyclopediatimeStudy begins here (baseline point)
47Epidemiologic Study Designs Randomized Controlled Trials (RCTs)the “gold standard” of research designsprovides most convincing evidence of relationship between exposure and effecttrials of hormone replacement therapy in menopausal women found no protection for heart disease, contradicting findings of prior observational studies.It is not unexpected to find that observational studies find different results than for clinical trials. For example there have been 100s of observational studies demonstrating that hormone replacement was protective for women. However, when this was put to a clinical trial, the surprising result was that hormone replacement was not protective
48Randomized Controlled Trials Understanding controlled trials: Why are randomised controlled trials important? by Bonnie Sibbald and Martin RolandDisadvantagesVery expensiveNot appropriate to answer certain types of questionsit may be unethical, for example, to assign persons to certain treatment or comparison groupsUnderstanding controlled trials: Why are randomised controlled trials important? By Bonnie Sibbald and Martin Roland
49Review QuestionsWhat designs can be used to describe the link between exposure and disease?Describe study design sequenceDescribe strengths and weaknesses of each design