> ## Documentation Index
> Fetch the complete documentation index at: https://confidence.spotify.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Rollouts

> A rollout is a way to gradually roll out a change or feature to a percentage of your users to ensure it doesn't have any negative impact on your users or your systems.

The distinguishing features of a rollout compared to other types of experiments
is that they:

* can only have two [treatment groups](/docs/capabilities/experiments/treatments)
* only use [guardrail metrics](/docs/capabilities/experiments/metrics#guardrail-metrics)
* have a variable allocation that lets you gradually roll out
* only use a [sequential design](/docs/capabilities/experiments/statistical-settings) where you see results continuously when the rollout is live

The main reasons for gradually rolling something out as opposed to
enabling it for all users immediately are:

* It lets you ensure that the experience you are rolling out is not worsening
  the user experience before you roll it out to everyone.

* See comparisons between the group that has the feature you are rolling
  out and the group that has the current default experience for any metric.

A rollout uses the same basic functionality as an A/B test.
It allocates users into two groups, a control group and a treatment group, and compares them.

The difference is that in a rollout, you want to roll out
a feature gradually. You might start with a 90/10 allocation with 10%
of eligible units in the treatment group to limit the impact of a potential issue. You then
monitor important metrics to see if you worsen the user experience. If the experiment is going
well, you increase the allocation to get more units in the treatment until you have rolled
it out completely.

## Understand Reach

Reach is the share of the total eligible population that receives the treatment. To monitor the
rollout, Confidence assigns an additional part of the eligible population to control.

With the default 90/10 treatment and control proportions, a rollout at 10% reach assigns 10% of
the total eligible population to treatment. Confidence assigns about 1.1% of the total eligible
population to control.

Monitoring can continue only up to the configured treatment share of the available allocation
space, which is 90% by default. Increasing reach above this threshold consumes the comparison
group and permanently disables monitoring.

## Related Resources

<CardGroup cols={2}>
  <Card title="Launch a Rollout" href="/docs/guides/roll-out-a-change">
    Step-by-step rollout tutorial
  </Card>

  <Card title="Guardrail Metrics" href="/docs/capabilities/experiments/metrics#guardrail-metrics">
    Configure rollout metrics
  </Card>

  <Card title="Monitoring" href="/docs/capabilities/experiments/monitoring">
    Monitor rollout health
  </Card>

  <Card title="A/B Tests" href="/docs/capabilities/experiments/ab-tests">
    Compare with A/B tests
  </Card>
</CardGroup>