# USMLE- Epidemiology and biostatistics II

rename
xogamire's
version from
2015-04-11 13:28

## 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 |

## 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) |

## 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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