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Epidemiology
Epidemiology
56
Medical
Graduate
12/28/2009

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Term
Which are the modes of transmission of an infectious disease?
Definition
1. Direct: person to person
2. Indirect: A) Common vehicle (e.g. single exposure, multiple exposure and continuous exposure). B) Vector.
Term
What are the components of the epidemiological triad of disease?
Definition
Host, environment, agent and vector
Term
What is carrier status?
Definition
When an individual is carrying an infectious disease but has no clinical or serologic evidence of infection.
Term
What is herd immunity?
Definition
It is the protection provided to susceptible individuals by non-susceptible individuals. This occurs because in this context there is a decreased likelihood of an infectious person to come in contact with a susceptible person. The level of immunity required for herd immunity to occur depends on the infectiousness of the disease. Some conditions for HI to occur are:
1. D mode of transmission is direct.
2. Infection creates good immunity.
3. D limited to single host.
It operates optimally with constant mixing of the population
Term
What is the fundamental assumption of infectious disease dynamic?
Definition
The number of new infections per unit of time is a function of the number susceptible and the number who are infectious
Term
What are the 3 critical elements of an outbreak investigation?
Definition
1. When did exposure occur?
2. When did disease began?
3. What is the incubation period?
If you have any 2 elements, the third can be estimated.
Term
What are the 9 criteria for causal association?
Definition
1. Temporality
2. Dose response.
3. Biologic plausability
4. If other hypothesis have been considered.
5. Strength of association
6. Compatible with current knowledge
7. Replication of findings
8. Cessation when exposure stopped
9. Gradient response
Term
What epidemiological method and measure of association is used to establish the risk when comparing multiple potential agents?
Definition
Cross-tabulation. The MA that is used is the Attack Rate.
Term
What is a rate?
Definition
How fast an event occurs. It requires time.
Term
What is a proportion?
Definition
It is a fraction of people affected by an event.
Term
What is incidence?
Definition
It is a rate that denotes the number of new events that occur in a specific period of time. It is a measure of RISK. It is used for etiology.
Term
What is Attack Rate?
Definition
It is a measure of risk. Number affected/number at risk in a specific period of time.
Term
What is prevalence?
Definition
It is the number of events at a point in time. It is not a measure of risk because the numerator has a mix of people with different duration of disease. It is an important measure of burden of disease.
Term
What is point prevalence?
Definition
It is the number of events at a specific point in time
Term
What is period prevalence?
Definition
It is the number of events at any point during a period of time.
Term
What is epidemiological surveillance?
Definition
It is the ongoing systematic collection, analysis and interpretation of health data that is essential for planning, implementation and evaluation of PH practice. This is closely integrated with a system for the timely dissemination of data.
Term
What is mortality rate?
Definition
It is a measure of risk. # of deaths/# people in pop at midyear. over a period of time.
Term
Prevalence is a function of...?
Definition
Incidence X duration of Disease
This is true in a steady state population, when migration in=to migration out.
Term
What is direct age adjustment and what is it used for?
Definition
Direct age adjustment is an epidemiological technique that is used to compare to different populations. Direct adjustment uses the data from the populations and applies it to a standard population, by doing this, the two differing populations are now comparable. This type of adjustment eliminates the effect of age, which is the single most important predictor of mortality.
Term
When is the mortality rate a good indicator of the incidence?
Definition
1. When CFR is high.
2. When duration of disease is short.
Term
What is indirect age adjustment?
Definition
It is an epidemiological technique used to compare 2 different population. With indirect Adjustment, we use the data form the population and will apply it to the sub-population. It is used when no age specific information is available for the sub-population. The Standard Mortality Ratio is obtained from dividing the Obs death/expected.
Term
What is the cohort effect?
Definition
It is a phenomena that is observed with mortality data over time. If you analyze cross-sectional data, you might end up with the wrong conclusion regarding the risk of death of a specific disease. It is important when observing mortality trends over time to think if the changes observed are given by the cohort effect.
Term
Age adjusted death rates are used to...?
Definition
Eliminate the effect of differences in age distribution in comparing death rates.
Term
What is validity?
Definition
It is the ability of a test to distinguish between diseased and non-diseased.
Term
What are some of the characteristics of sequential testing?
Definition
1. First test very sensitive.
2. Second test very specific.
By doing this, at the end, you will increase the net specificity (When compared to first test) and decrease the net sensitivity.
Term
What are some of the characteristics of simultaneous testing?
Definition
1. You will have increased net sensitivity.
2. Decrease in net specificity (when compared to both test).
Term
What are two factors that affect the predictive value?
Definition
1. The prevalence of disease in the population. The higher the prevalence, the higher the PPV.
2. When disease is rare, the specificity of the test being used.
2.
Term
What is reliability?
Definition
Can a test be repeated. This will depend of 3 factors:
1. Intrasubject variability (variation observed within individuals).
2. Intraobserver variability (var. of interpretation of results in a single individual).
3. Interobserver variability (variability among observer in interpreting results).
Term
What is the Kappa statistic?
Definition
It is a statistic that shows us the level of agreement between observers that is above of what would be expected by chance alone. It is usually accepted that a Kappa >0.75 represents excellent agreement and below 0.4 is poor agreement.
Term
What are the assumptions made when using a life table?
Definition
1. There are no secular (temporal) trends in effectivness of tx or in survivorship over calendar time.
2. Survival experience of those lost to follow up is the same as for those who stayed in the study.
Term
What is a RCT used for?
Definition
It is ideal to evaluate effectivness and side effects of a new intervention.
Term
Who was the first to conduct a RCT?
Definition
Pare in XVI century. It was not planned. Tx at the time for wounds (oil) vs a mixture of eg yolk, roses and turpentine.
Term
When was the first RCT conducted?
Definition
In the XVIII century. James Lind. Scurvy. 50 years after the experiment, the British Navy recommended citrus as part of the daily diet for seamen
Term
What are the advantages of using historical controls and when can they be useful?
Definition
This type of control can be used when a disease has a high mortality rate, and there is no tx for it, so hx control can be compared to px receiving new tx (still, temporal changes need to be rules out). It has to be considered that hx records were not collected for research, so information may be limited for this group. A solution is to use Simultaneous Non-randomized controls.
Term
What are some examples of problems with simultaneous non-randomized controls?
Definition
1. Captain and nausea pills.
2. BCG in cooperative vs non-cooperative.
3. Anti coagulant for those admitted to hospital on even number.
Term
Which type of design is the best for a clinical trial?
Definition
Randomization. The critical element of randomization is the unpredictability of the next assignment.
Term
How can we randomize?
Definition
1.Randomization table.
2. Sealed envelopes
Term
What is stratified randomization and what is it used for?
Definition
Because randomization does not guarantee that groups will be comparable, we can stratify groups based on factors that we know to be relevant (eg smoke, age, etc). By doing this, we are now certain that groups will have equal number of people with relevant factors and will increase the likelihood that groups are comparable. Randomization is made when we have the groups stratified.
Term
What are the two types of cross-over and what are their characteristics?
Definition
1. Planned: px assigned to one intervention. After being followed for certain period of time, it is measured and it moves to the other group. A px can be used as his own control. A concern is carryover, this is, that effect from tx A is carried to tx B. Decrease in enthusiasim with time.
2. Unplanned: People who change group that was assigned by randomization. We want to keep this to a minimum. A way to analyze this is doing the primary analysis by intention to treat, where you compare people based on how they were randmomized.
Term
What is a factorial study design?
Definition
Used with 2 different drugs that have 2 different outcomes and act independently. This is done in same group of px a the same time.
Term
What is needed to estimate a sample size in a RCT?
Definition
1. Alfa
2. Power
3. Delta: difference in response rates between groups.
4. An estimate of the response rate in one of the groups
4. One sided or two sided test.
Term
What is a ratio of risks?
Definition
It is the relative risk.
Term
Describe the phases involved in a RCT?
Definition
Phase 1: clinical and pharmacologic studies of 50-80 persons. Safety is addressed.
Phase 2: 100-200 persons. efficacy and relative safety.
Phase 3: >1000 persons. Usually multi-center. Safety and effectiveness.
Phase 4:post-marketing surveillance for adverse events.
Term
When doing a RCT, is the external validity increased?
Definition
No
Term
What is the major purpose of random assignment in a clinical trial?
Definition
Reduce selection bias in the allocation of treatment.
Term
In many studies examining the association between estrogens and endometrial cancer, a one sided significance test was used. the justification for using a one sided instead of 2 sided test was?
Definition
That researchers expected to find that estrogens were related to endometrial cancer
Term
What is an important reason when choosing a cohort study over a RCT?
Definition
In both designs, we go from exposure to disease. In the RCT, the exposure is usually an intervention that we suppose that is not harmful. When some exposure has been associated with a disease, we can use a cohort study to assess association between them. A very important difference, is that there is no randomization.
Term
What are the types of cohort study?
Definition
Prospective: individuals are followed in time.
Retrospective: We use past information, but we would still go from exposure (ascertained from records) to disease. It is less time consuming.
The design for both it's the same.
Term
Mention some of the most common biases observed in cohort studies
Definition
1. Bias in assessment of the outcome: when person making dx knows about the exposure status.
2. Information Bias: different info for exposed and non-exposed. More important in retrospective trials.
3. Biases due to non-response and lost to follow-up: if px with disease are lost to follow-up, we migh underestimate the IR.
4. Analytic Bias: from researchers having a preconception about the results.
Term
Why can cohort studies be impractical?
Definition
1. Time
2. When doing a retrospective cohort, quality of information might be bad or not existant.
3. Difficulties in assessing exposed and non-exposed.
4. Very expensive
Term
In a cohort study that studies the role of an exposure in the development of a disease, it is essential that:
Definition
Exp and non-Exp groups under study are similar with regard to identified confounding factors.
Term
What are some of the advantages of a prospective cohort design?
Definition
1. IR can be estimated.
2. Expo can be assessed.
3. Multiple outcomes can be evaluated.
4. Recall bias is minimized when compared to case control studies.
Term
Name some of the characteristics of retrospective cohort studies
Definition
1. Similar sample size as prospective.
2. Not useful for rare exposures.
3. Begin by defining exp and non-expo.
4. IR can be estimated.
Term
What is one of the major problems with non-randomization in cohort studies?
Definition
The possibility that a factor that led to the exposure, rather than the exposure itself is the cause of the disease.
Term
What is one of the mayor advantages of starting a cohort study by selecting the population and not be selecting expo an non-expo?
Definition
That we can study multiple exposures at the same time.
Term
What are some of the limitations of using hospital data?
Definition
1. Patients in the hospitals are selected in basis of: severity of disease, comorbidity, personal characteristics, admission policies.
2. Data collected not intended for research.
3. Population at risk difficult to define (denominator)
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