fix(experiments): mathematically correctly aggregate ratios like ACOS and ROAS before averaging

services/experimentAnalysis.ts

@@ -149,6 +149,31 @@ function splitPeriods(
 
 function avgMetric(data: ComputedWeeklyMetrics[], metric: string, durationWeeks: number): number {
   if (data.length === 0 || durationWeeks <= 0) return 0;
+
+  // For rates and ratios, we must sum the raw absolute components across all included weeks
+  // and THEN calculate the ratio, otherwise averaging percentages yields mathematically incorrect results.
+  if (metric === 'cvr') {
+    const totalUnits = data.reduce((s, w) => s + w.units, 0);
+    const totalSessions = data.reduce((s, w) => s + w.sessions, 0);
+    return totalSessions > 0 ? (totalUnits / totalSessions) * 100 : 0;
+  }
+  if (metric === 'ctr') {
+    const totalClicks = data.reduce((s, w) => s + w.clicks, 0);
+    const totalImpressions = data.reduce((s, w) => s + w.impressions, 0);
+    return totalImpressions > 0 ? (totalClicks / totalImpressions) * 100 : 0;
+  }
+  if (metric === 'roas') {
+    const totalRevenue = data.reduce((s, w) => s + w.revenue, 0);
+    const totalCost = data.reduce((s, w) => s + w.cost, 0);
+    return totalCost > 0 ? totalRevenue / totalCost : 0;
+  }
+  if (metric === 'acos') {
+    const totalRevenue = data.reduce((s, w) => s + w.revenue, 0);
+    const totalCost = data.reduce((s, w) => s + w.cost, 0);
+    return totalRevenue > 0 ? (totalCost / totalRevenue) * 100 : 0;
+  }
+
+  // For absolute quantities (units, revenue, sessions), we sum them and divide by the duration
   const sum = data.reduce((s, w) => s + getMetricValue(w, metric), 0);
   return sum / durationWeeks;
 }