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Epidemiologic Study Designs,[object Object],Chapter 4,[object Object]
Learning Objectives,[object Object],Define the general meaning of “study design”,[object Object],Distinguish descriptive from analytic study designs,[object Object],Discuss the rationale, strengths, and weaknesses of the various study designs,[object Object],Distinguish observational from experimental studies,[object Object],Define internal and external validity,[object Object],Identify the six primary activities and three types of meta-analysis,[object Object]
Study Designs,[object Object],At the heart of an investigation of any environmental health problem is the research or study design.,[object Object],A study design is the program that directs the researcher along the path of systematically collecting, analyzing and interpreting results,[object Object],It allows for descriptive assessment of events and for statistical inference concerning relationships between exposure and disease and defines the domain for generalizing the results,[object Object],Study designs in epidemiology are either implicit or explicit ,[object Object]
Descriptive Study Designs,[object Object],Descriptive study designs focus on assessing a sample without making inferences or causal statements.,[object Object],A descriptive study assists us in :,[object Object],1) providing information about a disease or condition,[object Object],2) providing clues to identify a new disease or adverse health effect,[object Object],3)identifying the extent of the public health problem,[object Object],4) obtaining a description of the public health problem that can be easily communicated,[object Object],5) identifying the population at greatest risk,[object Object],6) assisting in planning resource allocation,[object Object],7) identifying avenues for future research that can provide insights about an etiologic relationship between an exposure and outcome,[object Object]
Epidemiologic descriptive study designs,[object Object]
Epi chapter 4
Case Study,[object Object],The aim of a case study is to provide a complete understanding of a problem or situation,[object Object],It includes describing the characteristics of a person or group of people, the setting or community (cultural norms, community values, motives, etc.), the circumstance, and the problem,[object Object],Case studies are useful for obtaining an in-depth understanding of how and why questions,[object Object],The Centers for Disease Control and Prevention defines “cluster” as “an unusual aggregation, real or perceived of health events that are grouped together in time and space and that are reported to a health agency”,[object Object]
Epidemiologic evaluations of clusters have resulted in many triumphs in infectious and noninfectious disease control,[object Object],Well-known examples:,[object Object],The investigation of cases of pneumonia at the Bellevue-Stratford Hotel in Philadelphia in 1976,[object Object],The 1981 report that seven cases of Pneumocysticcarinii pneumonia had occurred among young homosexual men in Los Angeles,[object Object],Angiosarcomea among vinyl chloride workers,[object Object],Neurotoxicity and infertility in kepone workers,[object Object],Dermatitis and skin cancer in persons wearing radioactively contaminated gold rings,[object Object],Adenocarcinoma of the vagina and maternal consumption of diethylstilbestrol,[object Object],Phocomelia and consumption of thalidomide,[object Object],Salmonella meningitis in children,[object Object],Under one year of age in Taiwan,[object Object]
Cross-sectional Study,[object Object],A cross-sectional study involves simultaneous measurement of exposure and outcome factors at a specific point in time for a defined group of individuals.,[object Object],Cross-sectional studies may be thought of as a “snapshot” of at the frequency and characteristics of exposure and outcome factors in a group at a specific point in time,[object Object],Cross-sectional studies provides data that reflect the prevalence of an exposure or outcome in a selected group,[object Object]
Ecologic Study,[object Object],An ecologic study involves making comparisons between populations,[object Object],Ecologic studies  are often appropriate in environmental settings.,[object Object],Ex. Injuries are often associated with characteristics in the environment and may best be controlled be group-focused rather than efforts to change individual behaviors.,[object Object],In talking about an ecologic study, it is important to have an understanding of the concepts and rationale underlying this type of study.,[object Object]
Ecologic measures into three types:,[object Object],Aggregate measures-summaries of observations based on individual with a ,[object Object],Environmental measures-physical characteristics of a place such as a home or work site for members of a group.  Often individual exposure (or dose) levels remain unmeasured.,[object Object],Global measures-attributes of groups for which no analogue at the individual level exists,[object Object]
Type of ecologic analysis performed depends on whether the unit of analysis variables in the study is on the individual level or group level.,[object Object],If data are collected on the individual level, a value for  each variable is measured for every person in the study.,[object Object],If collected on the ecologic level, a value for each variable is measured for the group in the study,[object Object]
Research may involve completely ecologic analysis, partially ecologic analysis, or multi-level analysis:,[object Object],Completely ecologic analysis-the units of analysis on all variables are ecologic measures, such as the proportion exposed, the rate of injury, or the rate of disease.  The association may be made at the county level,[object Object],Partially ecologic analysis-a combination of individual-and group level data in the analysis.  For example, infant birth weight may be associated with environmental exposure to biogas from a landfill. ,[object Object],Multilevel analysis- a modeling technique that combines information at two or more levels.  For example, an individual-level analysis within each group could be performed.,[object Object]
In an analytic ecologic study, the ecologic association is assessed between the aggregate exposure level and the rate of the injury/disease for many groups,[object Object],The steps goes through to conduct an analytic ecologic study:,[object Object],1.  Develop a hypothesis regarding exposure and outcome,[object Object],2.  Define and select the ecologic units to be compared,[object Object],3.  Obtain  summary measures of exposure and outcome,[object Object],4. Plot the data for each ecologic unit of a graph,[object Object],5.  Determine if the hypothesized relationship exists,[object Object]
Time-trend designs can also be classified as exploratory or analytic (or time series), a comparison of injury/disease rates over it is made for a population in a single geographic area,[object Object],Age-period-cohort analysis is a special type of exploratory time-trend analysis.,[object Object],In an analytic time-trend analysis, the ecologic association between average exposure and change and disease rate change is made for a population in a single geographic area,[object Object]
Analytic Study Designs,[object Object],Analytic study designs are distinct from descriptive study designs I that they utilize a comparison group that has been explicitly collected,[object Object],Analytic study designs in environmental epidemiology are based on the comparison of individuals who are classified according to exposure and injury/disease status.,[object Object],The unit of analysis is the individual,[object Object],Analytic study designs fit into two general categories, observational and experimental,[object Object],In observational studies, researchers evaluate the strength of the relationship between an exposure and health-related state or event,[object Object]
In experimental studies some of the participants in the study are deliberately manipulated for the purpose of studying an intervention effect,[object Object],If the intervention being assessed is an environmental exposure being investigated for its adverse health effects, clearly it would be unethical to assign the intervention,[object Object],Most environmental investigations include both descriptive and analytic studies,[object Object],The descriptive study describes distributions of health-related states and events in the population,[object Object],The analytic study examines associations with the ultimate goal to establish cause-effect relationships,[object Object],Experimental studies typically come last because of their comparatively high cost, difficulty in execution, and narrow focus,[object Object]
Epidemiologic analytic study designs,[object Object]
Epi chapter 4
Experimental Study,[object Object],The randomized blinded controlled experimental study is considered to be the “gold standard” in epidemiology because it is an effective design for controlling confounding and bias,[object Object],Randomization of sufficiently  large numbers creates a situation where confounding by unmeasured variables is equally likely between groups,[object Object],Blinding the intervention eliminates unintended interventions  Blinding the outcome eliminates the ascertainment bias,[object Object],The strongest design is a between-group design where outcomes are compared between two or more groups of people receiving different levels of the intervention,[object Object],A within-group design may also be used where the outcome in a single group is compared before and after the assignment of an intervention,[object Object],The within-group design is susceptible to confounding from time-related factors such as climate-related conditions but may be adjusted for in the analysis,[object Object],Applying the experimental study design for assessing physical, chemical, biological, and psychosocial environments poses ethical challenges,[object Object]
Cohort Study,[object Object],In cohort studies, people are followed over time in order to describe the incidence or the natural history of a health outcome.,[object Object],As time passes, the number of potential health outcomes associated with a specific exposure increases,[object Object],Cohort studies are characterized by identifying exposure levels and then following over time, with the health outcome of interest assessed in the future,[object Object],Cohort studies are observational studies that have their unique strengths and weaknesses.,[object Object]
Cohort studies can be either prospective or retrospective.,[object Object],In a prospective cohort, the study begins by making measurements of exposure at baseline, following the cohort over time, and then measuring selected outcomes according to exposure status,[object Object],A retrospective cohort is the same as a prospective cohort except assembly of the exposure  and outcome variables all happened in the past,[object Object],In other words, a historical cohort is reconstructed with data on the exposure variable, measured in the past, and data on the outcome variable, measured in the past after some follow-up period,[object Object],The strength of a prospective cohort design is that is provides better control over exposure and outcome measures than in a retrospective cohort,[object Object]
In environmental epidemiology it is often convenient to compare cohorts from two separate populations,[object Object],It is also convenient to compare the outcomes for people exposed to a potential risk factor at a work site or location with people in the general population,[object Object]
Epi chapter 4
Case-Control Study,[object Object],In a case-control study, the presence of a potential risk factor is compared between those with and those without the health problem of interest.,[object Object],The study begins by identifying cases and controls and then investigating whether the cases were more likely to have been exposed than the controls,[object Object],A nested case-control study (case-cohort study) is a case-control study “nested” within a cohort study,[object Object],Levels of the risk factor are compared between cases and a sample of non-cases,[object Object],Yet to carry out a nested case-control study, samples or records of interest must be available from before the outcome condition occurred,[object Object]
Epi chapter 4
Case-Crossover Design,[object Object],Case-crossover design involves comparing the exposure status of a case immediately before its occurrence with that of the same case at a prior time,[object Object],It is appropriate where individual exposures are intermittent, the disease occurs abruptly and the incubation period for detection is short, and the induction period is short,[object Object],Individuals serve as their own controls, with the analytic unit being time-where the time just before the acute event is the “case” time compared with some other time, referred to as the “control” time,[object Object],Assumes that there are no confounding time-related factors, or that they are adjusted for in the analysis,[object Object],A time-related factor-is also assumed not to be present,[object Object]
Epi chapter 4
Case-crossover designs may also be used for injury research, but there are challenges,[object Object],1st-occupational injuries often require selected conditions to occur,[object Object],Hence identifying the person-time at risk, may be a challenge, and only a subset of an individual’s person-time at risk may be a considered for the study,[object Object],2nd-exposure information may not be available prospectively because most injuries are relatively rare, making prospective data collection inefficient in may settings,[object Object],Hence retrospective patient self-reporting of exposure may be necessary but it is prone to recall bias,[object Object],3rd-identifying control periods may be a challenge that is similar to the time when the injury occurs, yet for which the exposure is uncorrelated,[object Object]
Observational versus Experimental Study,[object Object],The observational design involves the science and planning of how information will be assembled, described, and related.,[object Object],In an observational study, researchers observe events for individuals in the study without altering them.,[object Object],The experimental design involves the science and planning of how an experiment will be conducted in order to obtain valid and reliable results.,[object Object],In an experimental study, researchers evaluate the effects fo an assigned intervention on an outcome.  ,[object Object],If an observational study design is selected, several questions should be answered:,[object Object],Should the observation be taken at a point in time or should the same subjects be studied over time ,[object Object],Should you begin with the outcome and then consider past events or begin with exposure and follow subjects forward in time to evaluate events that have not yet occurred,[object Object]
Observational studies are limited by confounding and bias, and basing causal inference on these studies is more difficult than with experimental studies,[object Object],Especially in environmental epidemiologic studies where the numbers are often small and the exposure is misclassified or poorly measured,[object Object],Cohort studies evaluating long-term exposure  may be ethical once a physical, chemical, biological, or psychosocial environment has been identified,[object Object],When the exposure is identified and people are informed of the environmental contaminant, people will naturally leave the polluted area and the study of duration is no longer feasible,[object Object]
Casual inference is strongest when based on experimental studies, followed by cohort studies,[object Object],The prospective cohort study is better than the retrospective cohort study, the nested case-control study, or the case-crossover study because there is better control over exposure and outcome measures,[object Object],Casual inference based on cohort and case-control studies is stronger than when based on cross-sectional or ecological studies because a comparison (control) group is involved,[object Object]
Study Subjects,[object Object],The study subjects are persons that are the object of scientific investigation,[object Object],Selecting the study subjects involves two primary decisions,[object Object],1st  decision-involves deciding,[object Object],Defining the study population essentially involves identifying those individuals best for answering the research question,[object Object],2nd decision involves how to obtain a sample of the population for descriptive and analytic evaluation,[object Object]
Epi chapter 4
Validity,[object Object],The validity of an investigation is that component of accuracy reflecting the level of systematic error in the study.,[object Object],Two types of validity:  external validity and internal validity.,[object Object],External validity refers to the extent the results of a study are relevant to people who are not part of the study (representatives),[object Object],Internal validity is the extent the results of a study are not attributable to bias or confounding,[object Object],A high response rate to questionnaires, minimized information bias, and proper control of potential confounders is often determined when designing the study.,[object Object]
Epi chapter 4
For example, incentives may ensure an adequate response rate, proper training may minimize interviewer bias, and information gathered on potential confounders at the time of data collection can allow for their adjustment in the analysis,[object Object],In environmental epidemiologic studies, samples of people selected for inclusion into the study are restricted by calendar time and geography,[object Object],Restriction by time, location, and other factors, as commonly done in environmental epidemiologic studies, minimizes the threat of confounding by these factors, thereby improving the internal validity of the study,[object Object]
Meta-Analysis,[object Object],It is the process or technique of synthesizing research results by using various statistical methods to retrieve, select, and combine results from previous separate but related studies,[object Object],Considered a more comprehensive or developed study because it is a systematic comparison of studies that results in the integration of several studies into one analysis.,[object Object],While a review summarizes similar studies, meat-analysis combines similar studies to analyze the aggregated information in order to identify consistency of studies, ,[object Object]
Meta-analysis is also used to aggregate subsets of studies, such as information based on age or sex.,[object Object],A meta-analysis of subsets is less successful in establishing causal relationships because meta-analysis does not allow the selection of subsets to be randomized.,[object Object],Meta-analysis of subsets does provide information in moderator variables,[object Object],Moderator variable can be thought of as a thrid factor that influences the direction or magnitude of a causal association between an exposure and outcome,[object Object]
For a meta-analysis to establish a causal relationship depends on both the research designs of each primary study and on the 6 methods of meta-analysis,[object Object],1.  Assessing evidence that a relationship or law, exists and that it is relatively non-artificial,[object Object],2.   Examining the form of a relationship,[object Object],3.  Analyzing the causal or key conceptual components of the independent variable and the key components of the dependent variable,[object Object],4. Assessing the scope of a relationship,[object Object],5. Analyzing the mediating processes that underlie a relationship,[object Object],6. Developing a model that counts for the variance among study outcomes,[object Object]
Researchers use one or many of these methods to accomplish 3 main outcomes or types of meta-analysis:,[object Object],1.  Integration of individual studies that all examined a single empirical relation ,[object Object],2.  Assessment of the generability of empirical relationships or associations,[object Object],3.  Examination of the effect of new theoretical concepts that had not been considered in the primary-level studies,[object Object]

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Epi chapter 4

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