Biostatistics for the MCAT

The MCAT doesn't ask you to compute heavy statistics, but it constantly asks you to interpret them from figures and tables. Know what each measure means and when it's being misused.

Describing data

• Mean is pulled toward outliers; median is robust. In a right-skewed distribution, mean > median.
• Standard deviation measures spread; standard error of the mean (SEM = SD/√n) measures precision of the estimate and shrinks with sample size.
• A normal distribution has ~68% of values within 1 SD, ~95% within 2 SD, ~99.7% within 3 SD.

Inference

• A p-value is the probability of observing data this extreme if the null hypothesis were true. p < 0.05 is the conventional threshold; it is not the probability the hypothesis is true.
• Type I error (α) = false positive (reject a true null); Type II error (β) = false negative; power = 1 − β rises with larger sample size and effect size.
• A 95% confidence interval that excludes the null value (0 for a difference, 1 for a ratio) corresponds to p < 0.05. Wide intervals mean imprecise estimates.