Optimizely records a **decision event** when a user is exposed to an experiment variation provided by Optimizely. Specifically, Optimizely logs a decision through the following sequence:
An experiment is **activated** via the Activate, Is Feature Enabled or Decide methods.
As a result, an SDK **decision event** is sent asynchronously to the Optimizely Event API. If the decision event indicates that the user is bucketed into an experiment, Optimizely logs the impression.
Decision events are used to generate the visitor counts shown on the [Results](🔗) page. They also serve as the denominator for measuring conversion rates and are used to determine your impression total.
Generally speaking, decision events are triggered automatically when you call Activate or Is Feature Enabled or Get Enabled Features for a user that Optimizely determines qualifies for a running experiment. No decision events are triggered for flag rollouts.
## How the SDK triggers impressions
### Decision events **are** sent
When the user is bucketed into a [feature test](🔗) via the Is Feature Enabled or Get Enabled Features method, regardless of whether the feature is enabled or disabled in the feature test variation.
When a user is bucketed into an A/B test via the Activate method.
A new visitor lands on a page and is exposed to an experiment.
A visitor refreshes a page and is exposed to the experiment again.
A return visitor is exposed to the experiment again.
### Decision events **are not** sent
Any time the Activate method returns `
null` because the user did not qualify for any experiment.
If the user is only bucketed into a feature rollout (via the Is Feature Enabled or Get Enabled Features method) and there is no running feature test associated with the feature
If neither a test nor rollout is running.
## Impressions deduplication
Depending on your application logic, your users may trigger rapid bursts of decision events. To ensure that your billing impression consumption is aligned to your end users' experience, Optimizely uses fixed-interval deduplication of decision events to calculate impression consumption. All decision events received within fixed 5-second intervals count as one impression for each user in an experiment. Optimizely buckets events into the fixed time windows based on when Optimizely received the event.
For more information on how impressions are triggered specifically, see [What is an impression in Optimizely](🔗).
## Impressions and billing
Starting September 2020, Optimizely has introduced a simplified usage billing component: Monthly Active Users (MAUs), which replaces impressions. Monthly Active Users (MAUs) is the number of unique users used in a decision or tracking event. It is a measure of the overall traffic where you are using the snippet, APIs, or SDKs to do something, specifically:
Personalization Campaign Evaluation
Feature Flag/Rollout Evaluation
Unlike impressions, it is not a measure of what percent of that traffic you are experimenting on - every user that gets evaluated is counted. This allows you to run large-scale experiments at 100% traffic in order to reach statistical significance more quickly.
See [What are monthly active users (MAUs) in Optimizely](🔗).