Block V, Epidemiology
Terms
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Define:
1. Primary prevention
2. Secondary prevention -
1. prevention of a disease before it occurs
2. disease is identified earlier in its course so we can delay or prevent further complications. - Regarding prevention: what is a population based approach?
- a preventative strategy designed for an entire population
- Regarding prevention: what is a high risk approach?
- a preventative strategy aimed at high risk idividuals
- what do physicians do with population based probability?
- they apply that model to their individual patients
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Define:
1. preclinical
2. subclinical
3. persistent
4. latent -
1. has the disease and will express itself in time
2. has the disease, never expresses itself
3. has the disease and cannot get rid of it for an extended period of time (may be whole life).
4. has the "genetic message" of the disease but not the organism itself. - what is the difference between the latent and carrier states?
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latent - has the genetic response to the disease but has no organism.
carrier - has the organism of the disease but has no genetic response (ie. no antibody response) - what are the three major components of disease risk?
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person
place
time - define "rate" and give an example of 3 commonly used rates in epidemiology
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rate - the rapidity at which a new event occurs
1. incidence
2. mortality rates
3. case-fatality rates - what is the formula for incidence?
- incidence = number of new cases during a given time period/number of people at risk of the given disease over the same time period
- what is the formula for prevalence?
- prevalence = # with disease at time X / # of people in given population at time X
- what is the relationship between prevalence and icidence?
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prevalence = incidence x duration of disease
P = I x D - what is the formula for annual mortality rate?
- annual mortality rate = # deaths from all causes in a given year / # people in the given population at midyear
- what is the formula for case-fatality rate?
- case fatality rate = # that died from the disease (over a given time, in a given population) / # who had the disease (over the given time, in a given population)
- what is the formula for proportionate mortality?
- proportionate mortality = # deaths from the disease (given time, given population) / # deaths total (given time, given population)
- which rate would I use to measure the severity of a disease?
- case fatality rate
- define sensitivity
- the likelihood that an individual who has the disease will test positive
- define specificity
- the likelihood that an individual that does not have the disease will test negative
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what test result would have these consequences?
- increased health cost
- increased patient and family anxiety
- labeling and possible insurability problems -
false positive
(no disease but test +) -
what test result would have these consequences?
- missed opportunity for treatment and possible cure
- false security -
false negative
(has disease but tests -) - what is the relationship between sensitivity and specificity?
- as you increase sensitivity, specificity decreases (and vice versa)
- what is two stage screeining used for?
- used to reduce the overall false positive test results
- what effect does two stage screeing have on sensitivity and specificity?
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sensitivity - decrease
specificity - increase - define positive predictive value
- the probability that a person who tested positive for the disease really has the disease
- define negative predictive value
- the probability that a person who tested negative for the disease really does not have the disease
- what is the effect of prevalence on PPV?
- increase prevalence, increase PPV
- what is the effect of specificity on the PPV?
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IF THE DISEASE IS RARE IN A POPULATION:
increase specificity, increase PPV - define intraobserver variation
- variation in two observations made by the same observer. (ie. radiologist reading same X-ray twice and having 2 different diagnoses)
- define interobserver variation
- two examiners derive different results (ie. second opinion)
- give an example of when an increase in survival time is not due to better treatment
- lead time bias - when the disease is diagnosed earlier there is a linger survival time
- in what scenario would case fatality rates not be usefel?
- in a chronic disease (where there is a long timespan between dianosis and death)
- what is the major weakness of 5-year survival calculations?
- lead time bias - this will cause a pseudo increase in survival #s
- define median survival time
- the length of time that the median number (half) of the people in the study survive
- define relative survival rate (using a formula)
- the observed survival in people with the disease / expected survival if disease were not present
- what is the name of the study type in which there is no control group?
- case study
- What is the major difference between a cohort study and a randomized control trial?
- RANDOMIZATION
- Define a cohort study (in general terms)
- an observational study that looks at exposed and non-exposed groups, then compares the incidence of the disease of interest between the two groups.
- what is the difference between a concurrent cohort study and a retrospective cohort study?
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concurrent - data is collected concurrently (as the study progresses forward)
retrospective - data is collected retrospectively - what are some major disadvantages of the cohort design? (5)
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1. expensive
2. time consuming (also have more loss to follow-up problems)
3. potential for outcome bias
4. large sample size needed
5. disease definitions can change over time - what are some advantages of the cohort study? (2)
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1. can establish temporal relationship between exposure and disease
2. can study association of exposure with many diseases -
cohort studies work best when:
1. the exposure is _________? (frequent/rare)
2. disease is __________ among exposed individuals? (frequent/rare)
3. time between esposure and disease is ___________ (short/long) -
1. rare
2. frequent
3. short - define a case-control study in general terms
- a study that compares CASES (people w/disease) to CONTROLS (people w/o disease)
- what is the strategy of a case-control study?
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- compare exposure rates of both groups and see if there is a difference
take "case" group, look at people that have the disease who were exposed. THen compare to the people exposed in the "control" group. - In case-control studies, you cannot determine the ________(prevalence/incidence) of the disease?
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prevalence
(we don't know all the people who were at risk and we might not have all the cases accounted for) - Give some examples of control samples taken from the "community"
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- neighborhood controls
- Random Digit Dialing
- best friends, sibs, spouses - What is one advantage and disadvantage of selecting a sample from the hospital?
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advantage - easy
disadvantage - ill-defined reference population - what is a confounder and what can researchers do to reduce the risk of confounders?
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confounder - a variable that skews study results
MATCHING can be used to reduce the risk of confounding variables - what is matching?
- selecting controls that are very similar to cases (regarding characteristics which are often associated with the disease in question)
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what is a:
1. practical problem with matching
2. conceptual problem with matching -
1. the more variables chosen to match the harder it is to find controls
2. when you chose to control for a variable by matching that variable can no longer be studied. - what is recall bias?
- a person with a disease will scrutinize their past in much more detail than a person without the disease. This leads to bias
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what is the advantage of using multiple controls:
1. of the same type?
2. of different types? -
1. increases power of study
2. allows us to explore alternative hypotheses, also allows us to explore potential biases - what are 4 advantages of case control studies?
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1. cheap
2. time efficient
3. easier to determine a link between exposure and disease
4. good for studying rare diseases - what is a nested case control study?
- a case control study that is "nested" into a cohort study design
- define a cross-sectional study (in general terms)
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studies prevalence:
exposure and disease are studied simultaneously - what are 2 disadvantages of cross-sectional studies?
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1. survival bias
2. can't determine a temporal relationship - define absolute risk
- the incidence of a disease in a population
- what two things help us to mathematically express whether there is an association or not between a certain variable and the disease in question?
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RR - relative risk
OR - Odds ratio - What are the differences between RR and OR?
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- RR tries to determine if there is an association between the exposure and the risk of disease
-OR tries to determine the probability of a disease in an "exposed" group vs. an "unexposed" group -
how would you interpret:
1. RR or OR =1
2. RR or OR <1
3. RR or OR >1 -
1. no association between exposure and disease
2. negative association between exposure and disease
3. positive association between exposure and disease - How does the OR differ between a cohort study and a case-control study?
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cohort study - odds of disease in exposed group/odds of disease in unexposed group
case-control study: odds that a case (disease person) was exposed/odds that a control (non disease person) was exposed - In which type of studies can RR be calculated directly?
- Cohort studies
- In which type of studies can RR not be directly calculated?
- case-control studies
- Under what condition is OR a fairly accurate estimation of RR?
- when disease is RARE
- what is attributable risk?
- incidence in exposed group-incidence in nonexposed group
- what is the formula for calculating %AR?
- %AR=incidence (exposed)- incidence (unexposed)/incidence (exposed) all x 100
- what would we use to determine, in a population, how much risk reduction of disease "X" we can expect if we eliminate exposure to "Y"?
- PAR - popoulation attributable risk
- what is the formula for calculating %PAR?
- %PAR = incidence in total population - incidence in nonexposed group / incidence in total population (all x 100)
- give 4 examples of types of variables used in medical research.
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1. nominal
2. dichotomous
3. ordinal
4. continuous -
what is nominal vs. dichotomous?
give an example of each -
1. nominal - categorical but no scale (blood groups)
2. dichotomous - a binary nominal variable (normal vs. abnormal) -
which variable is categorized with a scale?
(ie. 0, 1+, 2+, 3+, 4+ edema) - ordinal
- when working with inferential statistics what must you always state in your results?
- confidence interval
- what are the three measures of central tendancy?
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1. mean
2. median
3. mode - what is the mode?
- the most commonly observed
- what are the two measures of dispersion?
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1. range
2. percentiles - a quartile would be an example of what kind of dispersion measurement?
- percentiles
- what is the interquartile range?
- the measurement inbetween the 25th quartile and the 75th quartile (percentiles)
- what are three measures of dispersion based on the mean?
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1. mean deviation
2. variance
3. standard deviation - what is a type I (alpha) error?
- Conclude treatments are different but they really are not
- what is a type II (beta) error?
- conclude treatments are not different but they really are
- what is the difference between a standard error and a standard deviation?
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standard deviation: error for a sample
standard error: error for a population.