Biostatistics and Epidemiology Summary for USMLE Step 3
Biostatistics
Descriptive Statistics
- Mean, Median, Mode: Measures of central tendency.
- Standard Deviation (SD): Measures the dispersion of data points from the mean.
- Variance: The square of the standard deviation.
- Range, Interquartile Range (IQR): Measures of data spread.
Probability and Distributions
- Normal Distribution: Bell-shaped curve; mean = median = mode.
- Z-scores: Standardized scores to compare data points.
- T-distribution: Used when the sample size is small and/or the population variance is unknown.
- Chi-square Distribution: Used for categorical data to assess how likely it is that an observed
distribution is due to chance.
Inferential Statistics
- Hypothesis Testing: Null hypothesis (H0) vs. alternative hypothesis (H1).
- P-value: Probability of observing the data if H0 is true. P < 0.05 typically indicates statistical
significance.
- Confidence Intervals (CI): Range within which the true population parameter is expected to lie,
typically 95%.
- Type I Error (alpha): False positive; rejecting H0 when it is true.
- Type II Error (beta): False negative; failing to reject H0 when it is false.
- Power: Probability of correctly rejecting H0 (1 - beta).
Biostatistics and Epidemiology Summary for USMLE Step 3
Tests and Measures
- T-test: Compares means between two groups (independent or paired).
- ANOVA: Compares means among three or more groups.
- Chi-square Test: Compares observed vs. expected frequencies.
- Correlation and Regression:
- Pearson Correlation: Measures linear relationship between two continuous variables.
- Linear Regression: Predicts the value of a dependent variable based on the value of at least one
independent variable.
Epidemiology
Study Designs
- Cross-sectional Studies: Assess both exposure and outcome at a single point in time.
- Cohort Studies: Follow a group over time to assess the impact of exposure on the outcome.
- Prospective Cohort: Follows participants into the future.
- Retrospective Cohort: Uses past records to determine exposure status and follow outcomes.
- Case-Control Studies: Compare individuals with a condition (cases) to those without (controls) to
identify exposure history.
- Randomized Controlled Trials (RCTs): Participants are randomly assigned to experimental or
control groups to assess the effect of interventions.
Measures of Association
- Relative Risk (RR): Risk of an outcome in the exposed group divided by the risk in the unexposed
group.
- Odds Ratio (OR): Odds of exposure in cases divided by the odds of exposure in controls (used in
Biostatistics and Epidemiology Summary for USMLE Step 3
case-control studies).
- Attributable Risk (AR): Difference in risk between exposed and unexposed groups.
Bias and Confounding
- Selection Bias: Systematic differences in characteristics between those selected for the study and
those not.
- Information Bias: Errors in measuring exposure or outcome.
- Confounding: Distortion of the exposure-outcome relationship by a third variable.
Screening and Diagnostic Tests
- Sensitivity: True positive rate; the proportion of actual positives correctly identified.
- Specificity: True negative rate; the proportion of actual negatives correctly identified.
- Positive Predictive Value (PPV): Probability that subjects with a positive test truly have the disease.
- Negative Predictive Value (NPV): Probability that subjects with a negative test truly do not have the
disease.
Key Equations and Concepts
- Incidence: New cases / Population at risk over a specific time period.
- Prevalence: All cases (new and existing) / Population at a given time.
- Mortality Rate: Number of deaths / Population over a specific time period.
- Number Needed to Treat (NNT): 1 / Absolute Risk Reduction (ARR).
- Number Needed to Harm (NNH): 1 / Attributable Risk (AR).