What a 'Leaking' Segment Looks Like
Your "High-Value Customers" email segment has 2,400 contacts. You designed it to include customers who spent $500+ in the last 12 months. But your analyst built it 8 months ago and never updated the date filter. It now includes customers who spent $500+ 8–20 months ago — many of whom haven't bought since.
You're sending "loyal customer" messaging to people who may have already churned. The conversion rate on this "high-value" segment is mysteriously low. The segment is leaking.
The Three Types of Segment Leakage
1. Stale filters Date-based filters that aren't updated automatically. "Purchased in the last 90 days" written as a specific date range rather than a rolling window. Segments built on static lists rather than dynamic queries.
2. Missing exclusions Segments that don't exclude who they should. Your "new customer" segment includes people who made their second purchase last week. Your "lapsed" segment includes people who unsubscribed. Your "B2B" segment includes consumers who gave their work email.
3. Logic errors AND vs. OR conditions used incorrectly. "Customers who purchased Product A AND Product B" when you meant "Product A OR Product B." Nested conditions that don't behave as expected due to operator precedence.
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The Segment Audit Process
For each segment in your ESP or marketing platform:
Step 1: Document the intended definition What is this segment supposed to contain? Who should be in it? Who should definitely not be in it?
Step 2: Review the actual filter logic Open the segment builder and read every condition. Are date filters rolling or static? Are the AND/OR operators doing what you intended?
Step 3: Check the count How many contacts are in the segment? Is that count plausible given what the segment should contain? If your "active email subscribers" segment has more contacts than your total opt-in list, something's wrong.
Step 4: Spot-check records Pull 10 random contacts from the segment. Manually verify they meet the intended criteria. Pull 10 contacts who should be in the segment and verify they are.
Building Better Segments
- Always use dynamic/rolling date filters, not static date ranges
- Document every segment's intended definition in a comment or separate log
- Test new segments with a spot-check before sending to them
- Review all segments quarterly — conditions that were accurate at creation may not be accurate today
