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experimental designs

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experimenter has control over the exposure and participants are randomized

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Ecologic Study

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study in which the units of analysis are populations or groups. Comparing several different populations. information about both exposures (explanatory variables) and outcomes is collected. use group rather than individual because indiv. measurements might not be available but group ones are (archives, ) Common outcome variables: mortality, cause specific mortality, types of morbidity, etc.

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Intervention Study

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an investigation involving intentional change in some aspect of the status of the subjects ex. introduction of a preventive or therapeutic regimen or an intervention designed to test a hypothesized relationship randomized control trails quasi-experiments

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Cohort

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a population group or subset that is followed over a period of time. comparing the cases of individuals ex. Birth or age cohort (baby boom generation) work cohort (employers studied for occupational exposures) School/education Cohort

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Quasi-experimental study

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the investigator manipulates the exposure factor but does not randomly assign the individuals to groups.

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Observational studies

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lots of research. the investigator does not have control over the exposure factor. the experimenter is not able to randomly assign subjects

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Retrospective Cohort study

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one that makes use of historical data to determine exposure level at some baseline in the past. followup for subsequent occurrences of disease between baseline and the present is performed

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historical perspective study

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combines retrospective and prospective approaches

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Case control Study

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subjects are defined on the basis of the presence or absence of an outcome of interest. Cases = indiv. who have disease or outcome control group = do not have the disease can only examine a single outcome or a limited set of outcomes experimenter interviews cases and controls about past exposures

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Randomized control trails

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experiment in which subjects in the population are randomly allocated into groups (referred to as study and control groups) to receive or not to receive an experimental preventative or therapeutic procedure, or intervention. aka placebo. the results are accessed by rigorous comparisons of rates of disease, death, recovery, or other appropriate outcome in the study and control groups.

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Prospective Cohort study

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investigator starts with disease free groups for which exposures are determined first. the groups are then followed to see if development of disease will occur. EXPOSURES NEW CASES

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Case Control Advantages/Disadvantages

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A: 1. can be used to study low prevalence conditions 2. relatively quick and easy to complete 3. usually inexpensive 4. involve smaller number of subjects D: 1. measurement of exposure may be inaccurate 2. representativeness of cases and controls may be unknown 3. provide indirect estimates of risk 4. the temporal relationship between exposure factor and outcome cannot always be ascertained

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Cohort Studies Advantages/Disadvantages

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A: Permit direct observation of risk expose factor is well defined can study exposures that are uncommon in the population the temporal relationship between factor and outcome is known D: expensive and time consuming complicated and difficult to carry out subjects may be lost to followup during the case of the study exposures can be misclassified

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Ecologic Comparison study

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an assessment of the association between exposure rates and disease rates during the same time period.

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Matched case-control study

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the cases and the controls have been matched to a certain set of criteria such as age, sex, race, etc.

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Ecologic Study Advantages/disadvantages

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A: may provide information about the context of health can be performed when indv. measurements not available can be conducted rapidly and with minimal resources D: the ecologic fallacy imprecise measurement of exposure

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Seven factors that characterize study design

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1. who manipulates the exposure factor 2. How many observations are made? 3. what is the directionality of exposure? 4. What are the methods of data collection? 5. What is the timing of data collection 6. What is the unit of observation? 7. How available are the study subjects?

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Number of observations made

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sometimes, observations of subjects is only once. this is the approach of cross-sectional studies, many ecologic studies, and most case control studies. Sometimes more observations are made. Cohort studies and experimental studies

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Directionality of Exposure - Retrospective approach

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obtaining information about exposures that happened in the past. this method used in case control studies. investigator starts out with someone that already has the disease and asks them about exposures that may have led to disease

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Directionality of Exposure - A single point in time

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the study is referenced about a single point in time, in a survey. (used in cross sectional studies)

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Directionality of Exposure - Prospective Approach

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information about the study outcome is collected in the future. experimental studies and cohort studies

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Data Collection Methods

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some studies use only existing, previously collected data while other studies need to collect new data. Ecologic Studies use existing data

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Timing of Data collection

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in some studies information is collected about exposure from the past. if long periods of time have elapsed between the time the measurement of the exposure was taken and occurrence of disease, questions might be raised about the quality and applicability of the disease (person might not remember exposure to disease, therefore info is not valid) in other studies, subjects may be followed (into the future) over a period of time. If subjects drop out of the study the outcome variable may be lost.

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Unit of observation

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the individual or an entire group. most epidemiologic study designs use the individual as the unit of observation. ecologic studies use the group as the unit of observation

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Availability of Subjects

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Certain groups of subjects may not be available for research (ex. for ethical issues)

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Ecologic Correlation

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an association between two variables measured at the group level

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explanatory variables

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those studied as correlates of outcome variables ex. sex, socioeconomic level, age, income inequality, race, prevalence of physicians, unemployment, etc

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Census tract

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a small permanent statistical subdivision of a country, or equivalent. contains 1,000 and 8,000 people.

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Census Block

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an area bounded on all sides by visible and/or nonvisible features shown on a map prepared by the census bureau. smallest

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Ecologic Fallacy

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an erroneous inference that may occur because an association observed between variables on an aggregate level does not necessarily represent or reflect the association that exists on an individual level

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Odds ratio

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(an indirect measure of risk) a measure of the association between frequency of exposure and frequency of outcomes used in case control studies. the risk of an outcome associated with an exposure is estimated by calculating the odds of exposure among the cases and controls (AD)/(BC) = OR OR of more than one suggests a positive association when it is 1.0 there is no association

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Relative Risk

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the measure of association used in cohort studies. the ratio of the incidence rate (of the disease or health outcome) in an exposed group to the incidence rate (of the disease or health outcome) in a non exposed group. incidence = risk of occurrence of an outcome RR= (A/A+B)/(C/C+D)

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Attributable Risk

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in a cohort study refers to the difference between the incidence rate of a disease in the exposed group and the incidence rate in the non exposed group

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Population Risk Difference

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provides an indication of the benefit to the population derived by modifying a risk factor. This measure is the difference between the rate of disease in the non exposed segment of the population and the overall rate in the population. PRD = incidence in the total population - incidence in the non-exposed segment

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prophylactic trails

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designed to test preventative measures

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therapeutic trails

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evaluate new treatment methods

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clinical trail

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the research activity that involves the administration of a test regimen to humans to evaluate its efficacy and safety

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Single Blind Study/double blind study

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the participants don't know which group they are in. / the experimenters and the participants don't know

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Cross over design

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participants may be switched between treatment groups

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Community intervention (trails)

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is an intervention designed for the purpose of educational and behavioral changes at the population level. community trails are expensive, complex and time consuming

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Program evaluation

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the determination of whether the program meets stated goals and is justified economically

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External Validity

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ones ability to generalize from the results of the study to the external population some studies may select subjects by taking a sample of convenience - or by using random samples +

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sampling error

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type of error that arises when values obtained for a sample differ from the values of the parent population (well defined selection criteria)

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Internal Validity

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the degree to which the study has used methodologically sound procedures. ex. subjects need to be randomized, appropriate and reliable measurements need to be taken.

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Hawthorne effect

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participants behavioral changes as a result of their knowledge of them being in a study

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Recall Bais

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the fact that cases (participants in the study) may remember an exposure more clearly than controls

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Selection Bias

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distortions that result from procedures used to select subjects and from factors that influence participation in the study

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Healthy worker effect

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observation that employed populations tend to have lower mortality experience than the general population.

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Confounding

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the distortion of a measure of the effect of an exposure on an outcome due to the association of the exposure with other factors that influence the occurrence of the outcome.

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ABCD

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A- subjects with the disease that are exposed B- no disease, exposed C- with disease not exposed D- no disease, not exposed

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