Bucketing model

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The two-step allocation

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Step 1: allocation

hash1 = murmur3(salt + "/" + userId + "/Exposure") mod 10000
enter = hash1 < trafficAllocationBp

• salt, the experiment’s immutable UUID (its bucketingSalt). Every experiment gets its own salt at creation and it never changes.
• userId, the visitor’s stable id (UUIDv7 in localStorage + cookie).
• trafficAllocationBp, allocation in basis points (10000 = 100 %).

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Step 2: variant pick

hash2 = murmur3(salt + "/" + userId + "/Bucket") mod 10000
variant = the variant whose cumulative weight range contains hash2

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The hash

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Why the salt is per-experiment

1. A visitor that “always sees Variant A” would be wrong. If two experiments shared a salt, a visitor bucketed to variant 0 on test 1 would also be bucketed to variant 0 on test 2. That is a subtle correlation you almost never want. Per-experiment salts break that dependency.
2. Cloning generates a new salt on purpose. So a visitor that saw Variant B in the original does not automatically see Variant B in the clone. Cleaner reads on both.

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Where userId comes from

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Reproducing a bucket in Node

import murmurhash from 'murmurhash3js';

function bucket(salt, userId, trafficAllocationBp, weights) {
  const hAllocation = murmurhash.x86.hash32(salt + '/' + userId + '/Exposure') % 10000;
  if (hAllocation >= trafficAllocationBp) return null; // not in test

  const hVariant = murmurhash.x86.hash32(salt + '/' + userId + '/Bucket') % 10000;
  let cum = 0;
  for (const { key, weight } of weights) {
    cum += weight;
    if (hVariant < cum) return key;
  }
  return weights[weights.length - 1].key; // never happens if weights sum to 10000
}

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Mutual exclusion groups

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What could shift a visitor’s bucket

• Cleared cookies/localStorage. New id, new bucket. This is visible-to-user and expected.
• Cloned experiment. New salt, so re-bucketed. This is expected and desirable.
• Reset data on the experiment. New epoch, but same salt, the bucket does not shift.
• Weights changed on the experiment. For a visitor already inside, the variant they get depends on the new weights. See Variations for why weight changes on running experiments are gated.