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35 Cards in this Set
- Front
- Back
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Distinguishing characteristics: Directionality
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Directionality: When does the investigator observe the exposure variable relative to the time of observation of the health outcome?
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Distinguishing characteristics: Timing
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Has health outcome of interest already occurred before the study begins?
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Cohort study
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Cohort of subjects followed over time to measure the events that occur among exposed and non-exposed subjects
-Selected subjects are FREE of the disease of interest and identified on the basis of exposure and non-exposure at the start of the study -For causality you must have temporality |
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Directionality
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Forward
Backward non-directional |
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Timing
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Concerns the question of whether the health outcome of interest and therefore all study events have already occurred before the study begins
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Cohort study design
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Time- always moving forward
Population: start with disease free people See who is exposed or unexposed and follow forward to see who develops the disease |
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Cohort definition
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A group or aggregate of DISEASE FREE person who share a common attribute or experience and who are followed throughout their experience to observe the development or no development of a given health outcome.
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Cohort definition
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There are different types of cohorts
Cohort group members experience a common exposure associated with a specific setting -an occupational cohort -school cohort They share a non-specific exposure associated with a general classification -birth cohort |
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Examples of Cohorts
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-Persons born in the same year (birth cohort)
-Cohort exposed to radiation in 1945 -Person working at shell chemical Co. (1940-1989) -Cohorts exposed to a particular drug DES -Entire population (Cross-Classified-Framingham Cohort) |
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Distinguishing characteristics of Cohort studies
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1.) INCIDENCE
2.) looks for an association between two variables, comparing the incidence of a disease between two or more groups **Groups are formed based upon the level of the exposure variable observed by the investigator ***Because you have incidence you are looking at the risk of exposure |
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Types of Cohort Studies
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1.) Prospective Cohort study, (longitudinal study, concurrent)
2.) Retrospective Cohort study (Historical cohort study, non concurrent) 3.) Historical Prospective Cohort Study (Ambispective cohort study) |
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Retrospective cohort study
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The exposure still precedes the outcome
*Both exposure and disease have occurred before study is initiated -Requires existence of records -Direction is still forward, starting with exposure who are not diseased and following for a given time period to see if they develop the disease -More quickly done than a prospective cohort study -However, information on exposure or confounding variables not often available (e.g., smoking history, nutrient intake) -You can have many challenges because the data is already complete **Especially data on confounding! |
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Types of study populations
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1. A group that is not yet exposed...Study participants may represent a defined population before members "become exposed" or before their exposure has been determined (population-based sample)…i.e., Framingham, Nurses Health Study
2. A group that has both unexposed and exposed.The study participants may be selected because they represent exposed individuals and unexposed individuals (Exposure-based samples) -Examples: Radiologists vs. other specialty, BRCA mutation+ vs. BRCA mutation - |
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Advantages of cohort studies
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1. Measures incidence and thus permits direct estimation of disease risk
2. Does not rely on memory for exposure measurement; avoids bias due to selective recall 3. Because cohort studies begin with subjects free of disease, potential bias due to selective survival is eliminated 4. Can yield information on associations of exposure with several diseases |
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Disadvantages of cohort studies
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-Requires large samples to yield the same number of cases that can be studied more efficiently in a case-control study
-Particularly inefficient for studies of rare diseases -Direct observation of participants may cause changes in health behavior. |
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More disadvantages of cohort studies
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-Logistically difficult- long follow-up, often serious attrition of subjects
-possible bias in ascertainment of disease due to changes over time in criteria and methods -very expensive! |
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Analysis of Cohort studies
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1. Cumulative incidence
=# of new cases in given period/total population at risk 2. Incidence density =# of new cases in given period/total person-time of observation |
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Cohort measure of association
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Relative risk
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Case control studies
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Begin with group of diseased individuals (Cases) then people without disease (controls) are also selected for the study
-Study seeks to compare cases and controls with respect to their previous exposures |
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Purposes for Odds ratio include:
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Testing a hypothesis about an association between a disease and exposure.
e.g., the association between lung cancer and passive smoking Exploring data to identify exposures for further study: Data-driven hypotheses -tracking a point of origin a known infectious agent Rare disease…If you have an outbreak of diarrheal disease at a summer camp, you can use a case control to try and figure out which 1 or 2 sources might be most associated with disease. |
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Research question
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Is there an association between the presence or the absence of a particular disease or condition and one or more specific attributes (exposures/risk factors)?
OR- Among those with a disease is an attribute more or less prevalent than among those without the disease? |
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Issues in design of case-comparison studies
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Definition of cases:
-case definition should be as homogeneous as possible -need to have strict diagnostic criteria |
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Other issues in design of case-comparison studies
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Options for selection of cases:
-Hospital or medical care facilities (hospital based case comparison study) during a specified period of time. Relatively easy and inexpensive. -Defined population population based cased comparison study- select all affected individuals from a population or a sample of affected individuals. Avoids selection facts that lead an affected individual to utilize a particular health care facility or physician. A hospital could have selection bias- if you pick a hospital like MD Anderson, the study population could be people from all over the world. You need to make sure to identify who your source population is (specifically). |
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Other issues in design of case-comparison studies
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-incident cases
-prevalent cases 1.) must consider different issues that could affect your case-control study -characteristics of and source of cases -the need to obtain comparable information for cases and controls -practical and economic issues |
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Sources of controls
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The comparison group should be selected not to represent the entire non-diseased population population but the population of individuals who would have been identified and analogous to selection of cases source population!
-The comparison group should be comparable to the source population of the cases -Any exclusions or restrictions made in the identification of cases should apply equally to the comparison group and vise versa. If you are restricting smoking among cases, you must restrict smoking in your controls as well. |
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Sources of controls: Hospitals
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Subjects with conditions other than the disease under study
Advantages: 1.) easy to identify and readily available, thus reducing cost and effort 2.) more likely than healthy individuals to be aware of antecedent exposure or events (reduce potential recall bias) 3.) More likely to be comparable to cases on those factors that influence cases to have a particular health provider (SEP, Residency) |
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Sources of controls: Hospitals
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Disadvantages:
-May differ from healthy individuals in ways that may be associated with the disease or likelihood of hospitalization, therefore not representing the distribution of exposure in the population from which cases were derived. * studies have shown, hospitalized individuals are more likely to smoke, use oral contraceptives, and be heavier drinkers. -Source population often not identifiable because may be difficult to know which persons at risk of disease would go to a particular hospital if they developed disease. |
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Source of controls: General Population
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Households in the targeted neighborhoods
-may be suitable if source population is defined geographically -when neighbors are not source population fro cases, selection bias can result if source of cases is associated with exposure. Population registries (e.g., drivers license, voting lists) Disease registry list concerns parallel those for hospital-based controls. (the concern with the registries is selection bias) |
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Sources of controls: General population
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Random digit dialing
-Can approximate random sampling of households from source population, it that population is limited to residences with phones -households vary in number of people who reside in them and the amount of time people who are at home or available to answer the phone, which influences probability of contacting eligible controls -Difficulty distinguishing residential and commercial phone numbers can complicate calculating proportion of non responders **Becoming more difficult as cell phone only families |
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More Sources of controls: General population
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Advantages of using general population as controls:
*Healthy population that may better represent source of cases Disadvantages: *increased time, costs, differences in quality of the information, lower participation rates, participants may differ from non-participants |
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Sources of controls: Friends, Relatives, spouses
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-Result in individually-matched case-control sets, thus entails advantages and disadvantages of matching
-Friends may be likely to share exposures with controls (e.g., SEP, ethnicity, environmental exposures), likely to be cooperative -Exposures common to extroverts may be overrepresented -Decision about choice of controls is transferred from investigator to cases |
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Sources of controls: Dead
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-By definition, a dead control cannot represent the source population for cases since the individual is no longer at risk for the outcome.
-Dead controls will misrepresent the exposure distribution of the source population if exposure causes or prevents death to an important degree or is associated with another factors that does. -Use of dead controls is justified mainly for convenience e.g., in studies restricted to deaths, proportional mortality studies |
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Number of comparison groups
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-When there is concern about comparability of comparison group, it may be advantages to use several control groups.
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Number of comparison subjects
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-When sample is large, the optimal ratio for case and comparison subjects is 1:1
-When sample size is limited, with small numbers being available or when costs of obtaining information is greater for cases than for controls, the case to comparison group ratio can be altered to achieve the desired sample size ***As the number of comparison subjects per case increases, the power of the study increases; however, beyond 4 comparison subjects per case, there is very little benefit MULTIPLE CONTROLS INCREASE POWER! |
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Nested case-control and Case cohort studies
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-Cases are identified within a well defined cohort and consists of all individuals with outcome occurring in the defined cohort over a specified follow-up period.
-Controls may be selected from: 1.) Baseline cohort (case-cohort) 2.) Individuals at risk at time each case occurs (nested case-control) BOTH nested case control and case cohort reduce selection bias |
