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99 Cards in this Set
- Front
- Back
What are the characteristics of chronic diseases.
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1. Long period of symptoms
2. Residual disability and non-reversible pathologic changes 3. Special/long term rehabilitation 4. Indefinite period of treatment. |
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What are the difficulties in studying chronic diseases.
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1. There is no single agent
2. Long latent period 3. Indefinite onset 4. One set of factors are initiators, others are promoters |
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Differentiate between the different observational study designs.
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1. Cross-sectional
2. Case-control 3. Cohort |
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What are some characteristics of Cross sectional designs?
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1. Study groups are chosen-usually on a geographic basis.
2. Participants are chosen without regard to presence of factors or diseases 3. Presence of factors and diseases at the time of the study are ascertained. 4. They are used to build hypotheses. |
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What are some related concepts to Cross sectional designs?
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1. Prevalence studies (number of existing cases of a disease or health condition in a population)
2. Longitudinal studies 3. Dependent and independent variables |
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What are some advantages of a Cross sectional design?
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1. Simple, easy to interpret and common
2. fast, less effort, less expensive 3. relative risk calculations possible 4. estimates of prevalence possible 5. can explore many hypotheses regarding multiple factors and outcomes |
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Disadvantages of a Cross sectional design?
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1. not conclusive, you can't prove proper time sequence between exposure and disease.
2. can't calculate incidence 3. large samples needed 4. non-response bias |
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What are the characteristics of Case control studies?
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1. Identify those with disease first (case)
2.Identify a comparison group (controls) 3. Ascertain and compare factor exposure histories of both groups. |
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What are some related factors to Case Control study designs?
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1. Retrospective study design
2. used to test hypotheses |
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How do you choose cases for a Case control study?
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1. definite diagnostic criteria
2. Determine source of cases 3. Incidence or prevalence cases 4. must be representative of those with the diseas |
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How do you choose controls for a Case control study?
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1. The major methodological problem is finding appropriate comparison subjects.
2. Must come from the same source as cases. (Berksonian Bias) 3. Must be representative of those without the disease. |
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What is matching in a Case Control study?
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A technique in which controls are chosen who have the same characteristics as the cases
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What are characteristics of Matching?
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1. To reduce confounding factors
2. Group or individual matching 3. Level of "closeness" of a match 4. Choosing variables on which to match 5. Determine the number of variables on which to match. 6. Matched variables not included in analysis. |
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What are the advantages of a Case Control?
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1. More convincing evidence of causality
2. Fast, less expensive 3. Few subjects needed 4. good for rare diseases 5. can study many factors |
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What are some disadvantages of a Case Control?
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1. Likely confounding
2. recall bias 3. non-response bias 4. Can't get relative risk 5. can't get prevalence rate 6. can't get incidence rate |
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What are the sequence of a Cohort design?
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1. Choose cohort who have factor, but are not ill.
2. Choose comparison group (optional) 3. Track over time to determine disease rates in the two groups |
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How do you choose cohort's?
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1. Current of historical exposure
2. Define exposure 3. Must be representative of those exposed 4. Must be free of the disease(s) under study |
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Choosing a comparison group in a cohort study?
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1. Internal or external
2. Comparable to cohort group 3. Also must be free of the disease(s) under study |
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What is required in a follow-up cohort study?
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1. Requires co-operation of subjects
2. Followed for long period of time 3. Many outcomes can be observed |
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Advantages of a Cohort study?
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1. Most convincing and valid
2. Examines many outcomes 3. Can calculate incidence 4. Can calculate relative risk |
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Disadvantages of a cohort study?
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1. Attrition
2. Longer time (more expensive) 3. Large numbers needed 4. Observation bias |
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What is a cause?
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1. Merriam-Webster Dictionary: Something that brings about a result especially a person or thing that is the agent of bringing something about
2. KJ Rothman: An event, condition, or characteristic without which the disease would not have occurred 3. M Susser: Something that makes a difference |
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What are the five things to consider pertaining to "association"?
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1. Could the association be due to chance
2. Could the association be due to bias 3. Could the association be due to confounding 4. To whom does the association apply (representativeness) 5. Does the association represent a cause & effect relationship |
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Characteristics of a "cause"
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1. precede the effect (proximate vs. distant)
2. Can be either host or environmental factors (e.g., characteristics, conditions, actions of individuals, events, natural, social or economic phenomena) 3. Positive (presence of a causative exposure) or negative (lack of a preventive exposure) |
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What is the historic development of theories of causation?
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1. Divine retribution; imbalance in body humors caused by air, water, land, stars; spontaneous generation
2. Miasma: Disease transmitted by miasmas or clouds clinging to earth’s surface 3. Germ Theory of Disease and Henle-Koch Postulates: Most important postulate is that the microorganism must always be found with the disease. This postulate embodies the idea of specificity of a cause. That is, a one to one relationship between an exposure and a disease. 4. Web of Causation A paradigm for the causes of chronic diseases. Most important shift from Henle-Koch Postulates is the idea of multiple causes. Postulates were also revised for establishing causation in chronic diseases. 5. Recent Controversies Causation cannot be established. Causal criteria should be abandoned. |
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General model of causation: sufficient cause
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A set of conditions without any one of which the disease would not have occurred. (This is one whole pie.)
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General model of causation: Component cause:
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Any one of the set of conditions which are necessary for the completion of a sufficient cause. (This is a piece of the pie.)
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General model of causation: Necessary cause
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A component cause that is a member of every sufficient cause
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Attributes of a causal pie
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1. Completion of a sufficient cause is synonymous with occurrence (although not necessarily diagnosis) of disease.
2. Component causes can act far apart in time. 3. A component cause can involve the presence of a causative exposure or the lack of a preventive exposure. 4. Blocking the action of any component cause prevents the completion of the sufficient cause and therefore prevents the disease by that pathway |
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Causal guidelines suggested by Sir Ab Hill: Step one
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Strength of the Association:
The larger the association, the more likely the exposure is causing the disease. Example: Relative risk of lung cancer in smokers vs. non-smokers = 9; Relative risk of lung cancer in heavy vs. non-smokers = 20 |
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Causal "guidelines" suggested by Sir AB Hill: Step two
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Consistency
The association is observed repeatedly in different persons, places, times, and circumstances. Replicating the association in different samples, with different study designs, and different investigators gives evidence of causation. Example: Smoking has been associated with lung cancer in at least 29 retrospective and 7 prospective studies. Note: Sometimes there are good reasons why study results differ. For example, one study may have looked at low level exposures while another looked at high level exposures. |
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Causal "guidelines" suggested by Sir AB Hill: Step three
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Specificity
A single exposure should cause a single disease. This is a hold-over from the concepts of causation that were developed for infectious diseases. There are many exceptions to this. |
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Causal "guidelines" suggested by Sir AB Hill: Step four
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Temporality
The causal factor must precede the disease in time. This is the only one of Hill's criteria that everyone agrees with. Prospective studies do a good job establishing the correct temporal relationship between an exposure and a disease. Example: A prospective cohort study of smokers and non-smokers starts with the two groups when they are healthy and follows them to determine the occurrence of subsequent lung cancer. |
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Causal "guidelines" suggested by Sir AB Hill: Step five
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Biological gradient
A “dose-response” relationship between exposure and disease. Persons who have increasingly higher exposure levels have increasingly higher risks of disease. Example: Lung cancer death rates rise with the number of cigarettes smoked. Some exposures might not have a "dose-response" effect but rather a "threshold effect" below which these are no adverse outcomes. |
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Causal "guidelines" suggested by Sir AB Hill: Step six/seven
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Plausibility / Coherence
Biological or social model exists to explain the association. Association does not conflict with current knowledge of natural history and biology of disease. Example: Cigarettes contain many carcinogenic substances Many epidemiologic studies have identified cause-effect relationships before biological mechanisms were identified. For example, the carcinogenic substances in cigarette smoke were discovered after the initial epidemiologic studies linking smoking to cancer. |
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Causal "guidelines" suggested by Sir AB Hill: Step eight
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Investigator-initiated intervention that modifies the exposure through prevention, treatment, or removal should result in less disease.
Example: Smoking cessation programs result in lower lung cancer rates. Provides strong evidence for causation, but most epidemiologic studies are observational |
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Causal "guidelines" suggested by Sir AB Hill: Step nine
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Analogy
Has a similar relationship been observed with another exposure and/ or disease? Example: Effects of Thalidomide and Rubella on the fetus provide analogy for effects of similar substances on the fetus |
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What are the types of standardizing or adjusting?
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1. Stratified approach – Specific rates
2. Simple comparison with standard – Standardized Mortality Ratio 3. Direct (age) adjustment 4. Statistical adjustment – multivariate analysis |
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What is a crude rate?
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number of deaths / population * 100,000
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Why is age adjustment important?
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Allows for the comparison of data from populations of different age distributions
In different counties Over time (trends) |
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Why would you use a 2000 population standard?
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All mortality data since 1999 will be age-adjusted using the 2000 standard population
The HP2010 objectives will use age-adjusted rates based on the 2000 standard population |
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What are the other Benefits of Using the 2000 Standard Population
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CDC Wonder now allows for age-adjustment using the 2000 standard population
NCHS and the Chronic disease section of CDC will use the same standard for age-adjustments Easier to explain to the public Will no longer need to explain why some adjustments use the 1940 standard and others use the 1970 standard The rates adjusted to the 2000 standard million will be closer to the crude rate |
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'Trohoc" means...
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This disparaging term was given to case-control studies because their logic seemed backwards (trohoc is ?? spelled backwards) and they seemed more prone to bias than other designs.
No basis for this derogation. Case-control studies are a logical extension of cohort studies and an efficient way to learn about associations. |
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General definition of Case-Control study.
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A method of sampling a population in which cases of disease are identified and enrolled, and a sample of the population that produced the cases is identified and enrolled. Exposures are determined for individuals in each group.
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When is it desirable to conduct a case-control study?
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When exposure data are expensive or difficult to obtain
- Ex: Pesticide study described earlier When disease has long induction and latent period - Ex: Cancer, cardiovascular disease When the disease is rare Ex: Studying risk factors for birth defects When little is known about the disease Ex. Early studies of AIDS When underlying population is dynamic Ex: Studying breast cancer on Cape Cod |
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What kinds of cases should you use for a Case-Control study?
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Incident cases
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How can the SARS outbreak be described
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When SARS was first introduced, we knew the outcome, so we wanted to look at lots of risk factors to see what the risk factors were.
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What Kind of Cases To Use for a Case-Control study?
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Always use incident cases
Closer in time to action of the etiologic agent Determinants of survival are excluded Reduced confusion between cause and effect of disease, i.e. changes in smoking or drinking habits due to an illness |
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What do cases give you mathematically?
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Cases give you the numerators of the rates of disease in exposed and unexposed groups being compared:
Rate of disease in exposed: a/? Rate of disease in unexposed: c/? |
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What would you have if you were doing a cohort study?
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The denominators! If this were a cohort study, you would have the total population (if you were calculating cumulative incidence) or total person-years (if you were calculating incidence rates) for both the exposed and non exposed groups, which would provide the denominators for the compared rates.
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Where do you get the information for the denominators in a case control study?
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THE CONTROLS
A case-control study can be considered a more efficient form of a cohort study. Cases are the same as those that would be included in a cohort study. Controls provide a fast and inexpensive means of obtaining the exposure experience in the population that gave rise to the cases. |
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What is a control?
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Definition: A sample of the source population that gave rise to the cases.
Purpose: To estimate the exposure distribution in the source population that produced the cases. |
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How do you select controls?
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General population controls
Most often used when cases are selected from a defined geographic population Sources: random digit dialing, residence lists, drivers’ license records Example: Upper Cape Cancer Study |
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What are the advantages of selecting controls?
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Advantages of general population controls:
Because of selection process, investigator is usually assured that they come from the same base population as the cases. |
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What are the disadvantages of selecting controls?
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Disadvantages of general population controls
Time consuming, expensive, hard to contact and get cooperation; may remember exposures differently than cases |
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How do you obtain exposure History?
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Accessing data
Accuracy of self reported data, confidence in the data Recall Bias – those with a health problem recall health related events/exposures better Response Bias – those who respond are different from those who do not |
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Berksonian Bias is?
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If hospital controls are NOT from the same source as the cases, results in a selection bias
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Advantages to Hospital controls are?
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Same selection factors that led cases to hospital led controls to hospital
Easily identifiable and accessible (so less expensive than population-based controls) Accuracy of exposure recall comparable to that of cases since controls are also sick More willing to participate than population-based controls |
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Disadvantages to Hospital controls are?
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Since hospital based controls are ill, they may not accurately represent the exposure history in the population that produced the cases
Hospital catchment areas may be different for different diseases |
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What illnesses make good hospital controls?
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Those illnesses that have no relation to the risk factor(s) under study
Example: Should respiratory diseases be used as controls for a study of smoking and myocardial infarction? Do they represent the distribution of smoking in the entire population that gave rise to the cases of MI? |
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Selecting special control groups?
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Special control groups like friends, spouses, siblings, and deceased individuals.
These special controls are rarely used. Some cases are not able to nominate controls because they have few appropriate friends, are widowed, or are only or adopted children. Dead controls are tricky to use because they are more likely than living controls to smoke and drink |
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Important consideration for selecting controls: “the would criterion”
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Review: Controls are a sample of the source population that gave rise to the cases. Purpose is to provide information on the exposure distribution in the source population.
When selecting a control group consider the “WOULD CRITERION”: If a member of the control group actually had the disease under study WOULD he/she end up as a case in your study? Answer should be YES. |
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Fill in the blank...
Because controls are a sample of the _____ that produced the cases, _____ of the total population is often unknown Thus... |
population; size
Thus...you can’t get a cumulative incidence or incidence rate of disease or calculate the measures of association using the methods that we have learned. Instead you get a number called an odds which will function as a rate |
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Analysis of control studies.
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Definition of odds: the ratio of the probability of an event occurring to that of it not occurring
Example: Probability of getting a heads on one coin toss = ½ = .50. Probability of NOT getting a heads on one coin toss = ½ = .50. Odds of getting a heads on a coin toss = .5/.5 = 1:1 |
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Strengths of a case control study
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Efficient for rare diseases and diseases with long induction and latent period.
Can evaluate many risk factors for the same disease. So, good for diseases about which little is known |
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Weaknesses of case control study
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Inefficient for rare exposures
Vulnerable to bias because of retrospective nature of study May have poor information on exposure because retrospective Difficult to infer temporal relationship between exposure and disease |
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TP=true positive
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They had the disease
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FP=false positive
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Tested positive, but did not have the disease
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FN=false negative
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tested negative, but had the disease
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TN=true negative
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tested negative, but does have the disease
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Sensitivity
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probability that you have a positive test
P[+ test/disease] =TP/TP+FN |
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Specificity
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probability of a negative test given there is no disease
[P - test/no disease] TN/TN=FP |
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Predictive Value (positive)
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P [disease / a positive test]
TP/TP+FP |
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Predictive Value (negative)
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P [No diesease/ - negative test]
=TN/TN+FN |
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prevalence
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(a+c)/(a+b+c+d)
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accuracy
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(a+b)/(a+b+c+d) (usually compared to the gold standard)
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False Positive Rate
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b/(b+d)
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False Negative Rate
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c/(a+c)
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Specificity+false positive rate =
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1
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Sensitivity + false negative rate =
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1
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Draw out a Measure of Validity table
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True Diagnosis
Positive A B PPV Negative C D NPV Total sens. spec TOTAL |
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What is a cross-sectional study?
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Also termed prevalence study; it is a type of descriptive study designed to estimate the prevalence of a disease or exposure.
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What is a cohort?
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A group of individuals who share an exposure in common and who are followed over time; one example is an age cohort.
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What is a cohort study?
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A type of study that collects data and follows a group of subjects who have received a specific exposure. The incidence of a specific disease or other outcome of interest is tracked over time. The incidence i the exposed group is compared with the incidence in groups that are not exposed, have different levels of exposure, or have different types of exposures.
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What is the definition of Sensitivity?
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the ability of the test to identify correctly all screened individuals who actually have the disease. It is defined as the number of true positives divided by the sum of true positives and false negatives.
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What is the definition of Specificity?
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the ability of the test to identify only nondiseased individuals who actually do not have the disease. It is defined a the number of true negatives divided by the sum of false positives and true negatives.
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What is the definition of Predictive Value
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the proportion of individual screened positive by the test who actually have the disease.
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What is negative predictive value?
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an analogous measure for those screened negative by the test.
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What is the definition of reliability?
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Precision; the measuring of an instrument to give consistent results on repeated trials.
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What is the definition of validity?
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Accuracy; the ability of a measuring instrument to give a true measure.
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What is the key issue in community health assessment?
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Communtiy
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The idea is not competition, but ___________
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Collaboration; collaborative information
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When you're in a metropolitian area, you will see health facilities doing what?
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collaborating health assessment
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What should you include in assessing community life and needs?
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Everything...religion, education....everything!
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What should you emphasize in doing a community assessment?
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Community Ownership
Once the data is available, communities will become aware of their health problems. |
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Confidence Intervals: what does a 95% estimate
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It estimates the "true" population 95% of the time
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How are confidence intervals influenced?
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by the validity of the data and the sample size.
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P-value is
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the probability that the findings observed could have occurred by chance alone.
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True/False
Lack of statistical significance may be a reflection of insufficient statistical power to detect a meaningful association. |
True
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