Governing Equations

Contingency Table:
                Event+    Event-
  Drug+          a          b        a+b
  Drug-          c          d        c+d
                a+c        b+d       N=a+b+c+d

Proportional Reporting Ratio (PRR):
  PRR = [a/(a+b)] / [c/(c+d)]

Reporting Odds Ratio (ROR):
  ROR = (a/b) / (c/d) = a·d / b·c

Information Component (IC):
  IC = log2[(a+0.5)·N / (a+b+1)(a+c+1)]
  IC > 0 → positive signal

Chi-Square (Yates corrected):
  χ² = (|a·d - b·c| - N/2)² · N / [(a+b)(c+d)(a+c)(b+d)]

Signal Criteria (WHO):
  PRR ≥ 2, χ² ≥ 4, a ≥ 3
  All three must be met

Temporal Scan:
  Cumulative cases over time
  CUSUM or EWMA for early detection