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73 Cards in this Set

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What is prevalence?
# of existing cases in the population at a specific time
What is incidence?
# of new cases in the disease-free population that developed over a period of time
equation for prevalence
total # of cases at 1 pt in time / TOTAL population
equation for incidence
# of new cases over a given period / total population AT RISK
How are prevalence and incidence related?
P = I x average duration of disease (D)
When calculating incidence, how do you adjust the number for total population at risk?
remember to subtract the # of new cases from the total population at risk bc these individuals are no longer at risk
what kind of study can measure incidence?
cohort study

(longitudinal study --> follows a group of people who do not have the disease, but have risk factors)
what kind of study can measure prevalence
cross-sectional study

(cross-section in time)

AKA "prevalence study"
what happens to the prevalence as the mortality of a disease decreases
people live longer, so prevalence increases, bc the duration of the disease increased

(think HIV pts getting treated and living longer)

what is a cross-sectional study
people in a population are examined for the presene of a disease of interest at a given point in time
what are the advantages of cross-sectional studies
provide an efficient means of examining a population, allowing cases and noncases to be assessed all at once

they can be used as a basis for diagnostic testing

they can be used to plan which health services to offer and where
what are disadvantages of cross-sectional study
one cannot determine causal relationships bc information is obtained only at a single point in time

the risk or incidence of disease cannot be directly measured

sensitive tests rule OUT disease

specific tests rule IN disease
How do you calculate:

Define sensitivity.
probability that a patient with a disease will have a + test result

a sensitive test will rarely miss people
what is the false negative ratio of a sensitivity test
False Neg Ratio = 1 - Sensitivity
Define specificity.
probability that a patient without a disease will have a neg test

a specific test will rarely determine that someone has the disease when in fact they do not
what is the false positive ratio of a specificity test
False Pos Ratio = 1 - Specificity
What is the ideal diagnostic test in terms of sensitivity and specificity
the ideal test will be high in both
when is it desirable to have a high specificity diagnostic test
when there's a big penalty for missing the disease


when it is desirable to EARLY IN THE DIAGNOSTIC WORKUPR OR SCREENING TEST to reduce a broad differential

ie a diagnostic test for HIV/AIDS
when is a high specificity test desirable
to confirm a likely diagnosis


for situations in which the false positive result may prove harmful

ie a positive western blot confirmatory test for HIV
what is the purpose of predictive values
once a test result comes back, the results must be interpreted through use of predictive values (or posttest probabilities)
Define PPV.
the probability that a patient with a positive test result TRULY HAS the disease
How is specificity and PPV related?
the higher the specificity for a test, the higher it's PPV
how is disease prevalence related to PPV
the higher the disease prevalence, the higher the PPV of the test
Define NPV.
probability that a patient with a negative test result TRULY DOES NOT have teh disease
How is sensitivity of a test related to NPV
the more sensitive a test, the higher its NPV
How is disease prevalence related to NPV?
the lower the disease prevalence, the higher the NPV
because prevalence and predictive value are related, it is advantageous to apply diagnostic tests to what patient population?
diagnostic tests work better on individuals who have an increased likelihood of having the disease

(ie someone who is at-risk)
what happens to PPV and NPV when prevalence decreases
PPV decreases
NPV increases

remember that if prevalence is low, even a test with high sensitivity or specificity will have a low PPV
Besides predictive values, what's another way to measure the performance of a diagnostic test?
likelihood ratio (LR)
What is likelihood ratios (LR)?
In evidence-based medicine, likelihood ratios are used for assessing the value of performing a diagnostic test.

They use the sensitivity and specificity of the test to determine whether a test result usefully changes the probability that a condition (such as a disease state) exists.
what is the likelihood ratio of a positive test result
the probability of a person who has the disease testing positive

divided by the probability of a person who does not have the disease but tests positive
what is the likelihood ratio of a negative test result
the probability of a person who has the disease but tests negative

divided by the probability of a person who does not have the disease and tests negative
How does pretest odds, post test odds and likelihood ratio relate to each other?
The pretest odds of a particular diagnosis, multiplied by the likelihood ratio, determines the post-test odds.
How do you interpret the likelihood ratio in medicine?
A likelihood ratio of greater than 1 indicates the test result is associated with the disease.

A likelihood ratio less than 1 indicates that the result is associated with absence of the disease.
What does it mean for a likelihood ratio to be close to 1?
Tests where the likelihood ratios lie close to 1 have little practical significance as the post-test probability (odds) is little different from the pre-test probability, and as such is used primarily for diagnostic purposes, and not screening purposes.
How does the positive likelihood ratio change your odds of having a disease?
the likelihood ratio of a positive test shows how much the odds of a disease are increased if the test result is positive
How does the negative likelihood ratio change your odds of having a disease?
the likelihood ratio of a negative tests shows how much the odds of a disease are decreased if the test result is negative
What is pre-test odds?
In summary, the pre-test probability refers to the chance that an individual has a disorder prior to the use of a diagnostic test.

It allows the clinician to better interpret the results of the diagnostic test and helps to predict the likelihood of a true positive result.
Describe how people are selected and followed in a cohort study.
assemble a group of people who do not have the disease, but could potentially get it

for each risk factor for the disease, determine if the people in the group are exposed or unexposed

then follow the group over time and note the incidence of outcome events compared between the 2 groups
4 advantages of cohort studies
the follow the same logic as the clinic question
(if ppl are exposed, will they get the disease?)

they are the only way to DIRECTLY DETERMINE INCIDENCE
(bc they follow the cohort over time to assess disease development)

they can be used to assess the relationship of a given exposure to many diseases

in prospective studies, exposure is elicited without bias from a knowing outcome
3 disadvantages of cohort studies
they can be time consuming and expensive

studies assess only the relationship of the disease to a few exposure risk factors recorded at the start of the study

they require many subjects, which makes it difficult to study rare diseases
Are cohort studies prospective or retrospective studies?
they can be either one:

they can be prospective in which a cohort is assembled in the present and followed into the future

they can be retrospective, in which a cohort group is identified from past records and is followed to the present
In a cohort study, what statistical test can you calculate?
relative risk

by following a cohort group over time and observing for disease development, the researcher can determine the Relative Risk ratio
What does a case-control study analyze?
the population
How is a case-control study designed?
a series of CASES are identified and a set of CONTROLS are sampled from the underlying population
In a case-control study, what statistical test can you calculate/
you can estimate the frequency of exposure in the population at risk of the outcome

the researcher compares the frequency of exposure to a possible risk factor in the 2 groups
How do you select the case and control to minimize differences?
cases and controls should be comparable in terms of opportunity for exposure

they should be members from the same base population with an equal opportunity of risk factor exposure
What is confounding?
these are variables that influence the magnitude of a risk of getting a disease
How do you control a confounding variable in a case-control?
if somebody wanted to study the cause of myocardial infarct and thinks that the age is a probable confounding variable, each 67 years old infarct patient will be matched with a healthy 67 year old "control" person.

the easiest way is to exclude it
(ie if alcohol has a negative impact on heart disease, choose pts without alcoholism)
advantages of case-control studies
they use smaller groups than cohorts, thereby reducing cost

they can be used to study rare diseases and can easily examine multiple risk factorx
disadvantages of case-control studies
studies cannot calculate disease prevalence or incidence or directly estimate relative risk bc the numbers of subjects with and without a disease are determined artificially by the investigator rather than by nature

however, an odds ratio can be used to estimate a measure of relative risk (rate ratio)

retrospective data may be inaccurate owing to recall or survivorship biases
Define absolute risk.
its the incidence of disease
What is attributable risk (or risk difference)?
the difference in risk between the exposed and unexposed groups
Equation for attributable risk.
Attributable risk = incidence of disease in exposed - incidence in unexposed
Define relative risk (or risk ratio).
expresses how much more likely an exposed person is to get the disease in comparison to an unexposed person
What does relative risk say about the relationship between exposure and disease?
the relative risk indicates the relative strength of the association btw exposure and disease

this is useful when considering what is causing the disease
What is odds ratio?
this is just an ESTIMATE of relative risk that is calculated in case-control studies (bc it can't be calculated directly)
What does odds ratio say about one's exposure to a risk factor?
the odds ratio tells how much more likely it is that a person with a disease has been exposed to a risk factor than someone without the disease
What does odds ratio say about a person's chances of getting a disease in a case-control study?
in case-control studies, the odds ratio describes how many times more likely an exposed individual is to have disease compared to an unexposed individual
Equation for odds ratio.
Odds Ratio = odds that a disease person is exposed / odds that a nondiseased person is exposed
Define what the odds of something happening.
Odds = probability of event / 1 - probability of event

For example, the odds that a single throw of a die will produce a six are 1 to 5, or 1/5. The odds is the ratio of the probability that the event of interest occurs to the probability that it does not.

This is often estimated by the ratio of the number of times that the event of interest occurs to the number of times that it does not.
What is survival analysis?
it summarizes the average time from 1 event (eg diagnosis or start of tx) to any outcome that can occur only once during follow-up (ie death or recurrence of cancer)

Kaplan-Meier Curve
For a given Sn and Sp of a diagnostic test, and the PPV and NPV

what is the probability of having the disease if the test result is negative?
1 - NPV
For a given Sn and Sp of a diagnostic test, and the PPV and NPV

what is the probability of not having the disease if the test result is negative?
it's just NPV

the NPV tells you the chances that you do not have the disease if the test result is negative
For a given Sn and Sp of a diagnostic test, and the PPV and NPV

what is the probability of having the disease if the test result is positive?

the PPV tells you the chances of getting a disease if you tested positive

if you tested positive, the chances that you WILL NOT get the disease is 1 - PPV

What is lead-time bias?
when one makes an incorrect assumption that a "new screening test" improves prognosis and prolongs survival

the point is that the screening test was just performed at an earlier point in time, but the disease course was not changed

THINK: "a new screening test"

What is observer bias (aka ascertainment bias)?
refers to misclassification of an outcome and/or exposure

(eg labeling a diseased subject as non-diseased and vice-versa)

this happens bc the researcher placed a biased observation and made an assumption about it

What is the Hawthorne effect?
this the tendency of the study population to affect the outcome bc they are aware that they are being studied

to minimize hawthorne effect, keep pts unaware that they are being studied, but this is unethical

What is sample distortion bias?
when the estimate of exposure and outcome association is biased bc teh study sample is not representative of the target population
What is information bias?
there is an imperfect assessment of the association btw exposure and outcome due to errors in measurement of those exposure and outcome

minimize by using standardized techniques to measure