/configuring-experiment-rollout

name: configuring-experiment-rollout
description: Configures the rollout shape of a PostHog experiment — the variant split (50/50, 80/20, A/B/C ratios), the overall rollout percentage that gates how many users enter the experiment, and the disambiguation when a percentage like "roll out to 25%" could mean either. Use when the user mentions a rollout percentage, variant split, or traffic distribution; gives a ratio like 60/40, 70/30, or 80/20; asks "who sees the test variant?"; wants to increase, decrease, or change the rollout or split on a draft or running experiment; weighs equal vs uneven splits; or proposes a mid-experiment split change (often an anti-pattern that needs reset or end-and-restart).

Configuring experiment rollout

This skill answers: **Who sees what variant?**

Recommended approach: equal split + adjust rollout percentage

In most cases, experiments work best with an equal split. If you want to limit exposure to the test variant, adjust the rollout percentage instead.

Why equal splits are better:

• Equal splits maximize statistical power — each variant has the same sample size
• Equal splits balance traffic and thus reach significance faster
• Increasing user exposure throughout the experiment through increasing rollout is clean (changing split mid-experiment can cause users to switch variants, which is bad for user experience and data quality)

Always default to an equal split unless the user explicitly requests otherwise.

When an uneven split is required

Uneven splits combined with the default "Exclude from analysis" handling can introduce bias. If the experiment observes multi-variant users (users exposed to more than one variant) then those are dropped asymmetrically — the smaller variant loses a larger fraction of its assignments. If those users behave differently from the rest, the smaller variant's metrics will be skewed.

The right mitigation depends on experiment state:

1. **Pre-launch, or live but with few exposures so far — use an equal split and reduce the overall rollout.** Achieves the same test-variant exposure without the bias and preserves statistical power. See the disambiguation question below. 2. **Live experiment with significant exposures — switch multivariate handling to "First seen variant".** Changing the split mid-run reassigns users across variants (anti-pattern; see "Changing rollout on a running experiment" below). Switching handling instead keeps everyone in their original variant and avoids the asymmetric exclusion. See `configuring-experiment-analytics` for how to set this. Note that "first seen" handling can introduce other biases, but it's preferable to mid-run reassignment.

The two rollout controls

There are two separate controls that determine who sees what. Both live on the linked feature flag, sent through the `feature_flag` object in the flag's own shape (not the deprecated `parameters` keys).

1. Variant split (`feature_flag.filters.multivariate.variants`)

How users **inside** the experiment are distributed across variants.

• Array of `{key, name, rollout_percentage}`, where the `rollout_percentage` values must sum to 100
• Minimum 2 variants, maximum 20
• No specific variant key is required — the analysis baseline defaults to the variant keyed `"control"` when present, else the first variant
• Default: control 50% / test 50%

If the user says "A/B/C test" without naming keys, key the baseline `"control"` (the convention) and create additional variants for the others; if they ask for specific keys, use them as-is with the baseline first.

2. Overall rollout (`feature_flag.filters.groups[0].rollout_percentage`)

What percentage of **all** users enter the experiment at all, sent as a single rollout group: `groups: [{ "properties": [], "rollout_percentage": N }]`. Default: 100%.

Users not included are excluded entirely: they don't see any variant and are **not part of the analysis**.

Where these are sent

Both controls live inside `feature_flag.filters`:

{
  "feature_flag": {
    "filters": {
      "multivariate": {
        "variants": [
          { "key": "control", "name": "Control", "rollout_percentage": 50 },
          { "key": "test", "name": "Test", "rollout_percentage": 50 }
        ]
      },
      "groups": [{ "properties": [], "rollout_percentage": 100 }]
    }
  }
}

`filters` may also carry `aggregation_group_type_index` (to run the experiment on a group type rather than individual users) and `payloads` (JSON-encoded strings keyed by variant key). On a **running** experiment, any flag-config change must also send `update_feature_flag_params: true`, otherwise the API rejects the update before it reaches the flag (see "Changing rollout on a running experiment").

How they interact

These two controls multiply:

| Overall rollout | Variant split | % seeing test | % in analysis | | --------------- | ------------------ | ------------- | ------------- | | 100% | 50/50 | 50% | 100% | | 100% | 75/25 control/test | 25% | 100% | | 50% | 50/50 | 25% | 50% | | 25% | 50/50 | 12.5% | 25% |

The disambiguation question

**CRITICAL**: If the user requests an uneven variant split (e.g. "60/40", "70/20/10") or mentions a specific percentage that could refer to either the split or the rollout (e.g. "roll out to 25%"), you MUST clarify before proceeding. This covers two cases:

Case 1: Single percentage ("25%", "roll out to 40%")

The percentage is ambiguous — it could mean a variant split or a rollout change. Ask:

> There are two ways to get 25% of users seeing the test variant: > > 1. **Reduced rollout with equal split** (recommended): reduce the overall rollout and split > variants equally. Only a subset of users enter the experiment, and of those, each variant > gets the same share