fix(experiments): use exact fractional proportions for overlapping weeks in DiD calculation

services/experimentAnalysis.ts

@@ -40,6 +40,7 @@ interface ComputedWeeklyMetrics extends WeeklyMetrics {
   cvr: number;
   ctr: number;
   roas: number;
+  fraction?: number;
 }
 
 function getISOWeekStart(year: number, week: number): number {
@@ -142,9 +143,39 @@ function splitPeriods(
     // We prioritize assigning to 'after' so that we capture all units happening 
     // closely around the experiment dates for accurate reporting.
     if (overlapsAfter) {
-      after.push(w);
+      const overlapStart = Math.max(weekStartTs, startTs);
+      const overlapEnd = Math.min(weekEndTs, endTs);
+      const overlapFraction = Math.max(0, Math.min(1, (overlapEnd - overlapStart) / (7 * 86400000)));
+
+      if (overlapFraction > 0) {
+        after.push({
+          ...w,
+          units: w.units * overlapFraction,
+          revenue: w.revenue * overlapFraction,
+          sessions: w.sessions * overlapFraction,
+          cost: w.cost * overlapFraction,
+          clicks: w.clicks * overlapFraction,
+          impressions: w.impressions * overlapFraction,
+          fraction: overlapFraction
+        });
+      }
     } else if (overlapsBefore) {
-      before.push(w);
+      const overlapStart = Math.max(weekStartTs, beforeStartTs);
+      const overlapEnd = Math.min(weekEndTs, beforeEndTs);
+      const overlapFraction = Math.max(0, Math.min(1, (overlapEnd - overlapStart) / (7 * 86400000)));
+
+      if (overlapFraction > 0) {
+        before.push({
+          ...w,
+          units: w.units * overlapFraction,
+          revenue: w.revenue * overlapFraction,
+          sessions: w.sessions * overlapFraction,
+          cost: w.cost * overlapFraction,
+          clicks: w.clicks * overlapFraction,
+          impressions: w.impressions * overlapFraction,
+          fraction: overlapFraction
+        });
+      }
     }
   }
 
@@ -154,7 +185,8 @@ function splitPeriods(
 function avgMetric(data: ComputedWeeklyMetrics[], metric: string): number {
   if (data.length === 0) return 0;
   const sum = data.reduce((s, w) => s + getMetricValue(w, metric), 0);
-  return sum / data.length;
+  const totalFraction = data.reduce((s, w) => s + (w.fraction || 1), 0);
+  return totalFraction > 0 ? sum / totalFraction : 0;
 }
 
 const METRICS = ['units', 'sessions', 'cvr', 'ctr', 'roas', 'revenue', 'acos'];