USMLE Step 3 Biostats Notes


  • (+) Likelihood ratio = sensitivity / 1-­‐specificity = likelihood of having the disease given a positive result. This is different from PPV in that PPV is prevalence dependent.
  • (-­‐) Likelihood ratio = 1-­‐sensitivity/specificity = likelihood of not having the disease after a test result comes back negative. NPV, in contrast, is prevalence dependent.
  • PPV increases with increased specificity. NPV increases with increased sensitivity. Therefore a test with the highest PPV will have the highest specificity. A test with the highest NPV will have the highest sensitivity.
  • Higher prevalence increases PPV and decreases NPV. Lower prevalence decreases PPV and increases NPV.
  • Nominal data is dichotomous and only has two categories (e.g., male vs female).
  • Ordinal data has ranking but no numerical value (e.g., freshman, sophomore, junior, senior).
  • The median is a better indicator of central tendency (vs mean) in data with a highly skewed distribution.
  • Hazards Ratio:
    • Measure of how much effect something actually had.
    • Value of 1.00 means there is no difference between the two groups.
    • A ratio < 1 indicates a protective effect, and > 1 indicates a detrimental effect.
    • If the confidence interval of the hazard ratio includes 1.00 (null value), then the effect wasn’t statistically significant.
    • If the interval doesn’t include the value, the difference was significant.