> For the complete documentation index, see [llms.txt](https://docs.mediarithmics.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.mediarithmics.io/audience-and-activation/build-your-audiences/segment-reference/segment-types/partition-segment.md).
# Partition segment
The random, mutually exclusive split of your datamart into parts of roughly equal size.
## What it is
Partitioning your datamart means dividing your collection of users into **Partition segments** of roughly equal size. Every user is randomly attributed to one, and only one, partition segment within a partition.
{% hint style="info" %}
By default, your datamart comes with a partition of 100 partition segments - so each one holds roughly 1% of your users, and a group built from 10 of them is roughly a 10% sample.
{% endhint %}
## Why they exist
Partitions are the platform's source of unbiased random samples, which makes them the substrate for two things:
* **A/B testing.** Expose some partition segments to a campaign, hold the others back, and compare conversion rates. Simple randomisation in an [experiment](/audience-and-activation/build-your-audiences/segment-reference/segment-types/experiment-groups-segment.md) works exactly this way, and the permanent control group configured at datamart level is built from partitions too - both are documented in the **Measurement & Attribution** section, under *Set up an A/B test*.
* **Lookalike work.** A partition can be used as the starting point for building a larger audience.
## Creating a partition
Partitions are managed in datamart settings, under **Settings > Datamart > Partitions**, where existing partitions are also listed.
{% stepper %}
{% step %}
### Create the partition
Click **New Partition**, then **Random Split**.
{% endstep %}
{% step %}
### Size it
Define a **Partition Name** and a **Part Count**, from 0 to 100.
{% endstep %}
{% step %}
### Save
Save the partition. It is displayed alongside any partitions created earlier.
{% endstep %}
{% endstepper %}
{% hint style="warning" %}
A partition is **dynamic** by nature: users are liable to move in and out of it over time. Anything built on partitions inherits that, which is why a group drawn from them is never a frozen list.
{% endhint %}
## Related pages
Experiment groups segment
What simple randomisation builds out of partitions.
---
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