Real estate agents were given identical information about a house, including a listing price that was either high or low. The "expert" agents' valuations were pulled 41% of the way toward the anchor. They claimed the listing price had no effect on their professional judgment. It had a 41% effect.

The Framework

The anchoring index quantifies how much an anchor influences a judgment: (anchored estimate − control estimate) ÷ (anchor value − control estimate) × 100. An anchoring index of 100% means the estimate matches the anchor perfectly. An index of 0% means the anchor had no effect. Real-world indices typically range from 30-60%, meaning the anchor moves judgments roughly a third to half of the distance between the unanchored estimate and the anchor value.

The power of the index is diagnostic: it allows you to measure and compare anchoring effects across contexts, populations, and expertise levels. The finding that real estate agents showed indices of 41% — nearly identical to students with no real estate expertise — demolished the claim that expertise protects against anchoring.

Where It Comes From

Kahneman introduces the anchoring index in Chapter 11 of Thinking, Fast and Slow as a measurement tool for quantifying anchoring's magnitude. The real estate study (Northcraft and Neale, 1987) used the index to show that professional expertise provided essentially zero protection against anchoring effects.

> "You should assume that any number that is on the table has had an anchoring effect on you." — Thinking, Fast and Slow, Ch 11

Cross-Library Connections

Voss's Ackerman system in Never Split the Difference is designed to maximize the anchoring index in your favor: an extreme opening offer (65% of target) creates the largest possible gap between the anchor and the counterpart's initial expectation, pulling their final estimate as far as possible.

The Implementation Playbook

Negotiation Preparation: Calculate your desired anchoring index before any negotiation. If the counterpart's likely estimate is $100K and your target is $80K, anchoring at $60K creates a $40K gap. A 40% anchoring index would pull their estimate to $84K — close to your target. The more extreme the anchor (within credibility limits), the higher the index.

Pricing Research: Measure your own anchoring index. Present yourself with an anchored question and an unanchored question about the same product. The gap between your two answers is the anchor's effect on you — and knowing it is the first step to correcting for it.

Defense Against Anchoring: When someone presents a number, pause and generate your own estimate independently before engaging with theirs. The anchoring index can be reduced (though never eliminated) by active counter-anchoring: generating an extreme number in the opposite direction before deliberating.

Key Takeaway

The anchoring index proves that anchoring is not a vague "influence" — it's a measurable, quantifiable effect that moves expert and novice judgments by roughly the same magnitude. Knowing the index means you can predict — and design — the magnitude of any anchoring intervention.

Continue Exploring

[[Anchoring (Dual Mechanism)]] — The two mechanisms (adjustment + priming) that produce the index

[[Price Anchoring]] — The applied anchoring framework across 5 books

📚 From Thinking, Fast and Slow by Daniel Kahneman — Get the book