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USMLE- Epidemiology and biostatistics II

rename
soxowelu's version from 2016-08-05 19:22

Section 1

Question Answer
How can you do an evaluation of a diagnostic test?Use a 2X2 table, you then have true positive (TP), false positive (FP), true negative (TN) and false negative (FN)
What is the definition of sensitivity?Proportion of all people with disease who test positive.
what are the mnemonics for the sensitivity?PID= Positive In Disease and SNOUT= SeNsitivity rules OUT (because 100% of sensitivity is desirable for ruling out disease)
What are the equations for the sensitivity?Sensitivity= TP/ (TP+FN) and Sensitivity= 1- false-negative rate
What is the definition of specificity?Proportion of all people without disease who test negative
What are the mnemonics for the specificity?NIH= Negative In Health and SPIN= SPecificity rules IN
What are the equation for specificity?specificity= TN/(TN+FP) and specificity= 1- false-positive rate
What means positive predictive value (PPV)?proportion of positive test results that are true positive (probability that person actually has the disease given a positive test result)
What is the equation for the PPV?PPV=TP/(TP+FP) and a high pretest probability (high prevalence)=>High PPV
What means negative predictive value (NPV)?proportion of negative test result that are true negative (probability that person actually is disease free given a negative test result)
What is the equation of NPV?NPV= TN/(FN+TN) and a high pretest probability (high prevalence)=>low NPV
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Section 2

Question Answer
What is the mnemonic for incidence?Incidence looks at new incidents
What is the mnemonic for prevalence?prevALence looks at ALL current cases
What is the equation of the incidence rate?incidence rate= (#of new cases in a specified time period)/(population at risk during same period of time)
What is the equation of prevalence?prevalence=(# of existing cases)/(population at risk)
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Section 3

Question Answer
In what type of studies do we use odds ratio (OR) and what is the formula to measure it?often used in case-control studies, OR=(a*d)/(b*c)
In what type of studies do we use relative risk (RR) and what is the formula to measure it?typically used in cohort studies. RR= (a/(a+b))/(c/(c+d))
What means attributable risk and what is the formula to measure it?the difference in risk between exposed and unexposed groups (proportion of disease occurrences that are attributable to the exposure) and attributable risk = (a/(a+b)) - (c/(c+d))
What is the definition of absolute risk reduction (ARR)?absolute reduction in risk associated with a tx as compared to a control
What is the ARR if I say: 8% of people who receive a placebo vaccine develop flu vs 2% of people who receive a flu vaccine.ARR=8%-2% ==> ARR=6%
What means number needed to treat and how can we calculate it?Number of patients who need to be treat for 1 patient to benefit. NNT=1/ARR
What means number needed to harm and how can we calculate it?Number of patients who need to be exposed to a risk for 1 patient to be harmed. NNH=1/attributable risk
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