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

# Quasi-constant features

> Learn how to use the quasi-constant features test

## Definition

The quasi-constant features test allows you to specify if a certain feature is expected
to be near-constant (low variance) or not.

## Taxonomy

* **Task types**: Tabular classification, tabular regression.
* **Availability**: <Tooltip tip="Continuously evaluate your models and datasets as you iterate on their versions.">development</Tooltip>
  and <Tooltip tip="Monitor a model in production, measure its health, check for drifts and set up alerts.">monitoring</Tooltip>.

## Why it matters

* Quasi-constant features have very low variance and tend to not be as useful for models.
* If a feature shouldn't be quasi-constant but is, you might want to re-normalize it.

## Test configuration examples

If you are writing a `tests.json`, here are a few valid configurations for the character length test:

<CodeGroup>
  ```json Development theme={null}
  [
    {
      "name": "Feature 'Year' is not quasi-constant",
      "description": "Asserts that the feature 'Year' does not have a very low variance",
      "type": "integrity",
      "subtype": "quasiConstantFeature",
      "thresholds": [
        {
          "insightName": "featureProfile",
          "insightParameters": [
              {"name": "name", "value": "Year"} // Selects feature `Year`
          ],
          "measurement": "isQuasiConstant",
          "operator": "is",
          "value": False
        }
      ],
      "subpopulationFilters": null,
      "mode": "development",
      "usesValidationDataset": true, // Apply test to the validation set
      "usesTrainingDataset": false,
      "usesMlModel": false,
      "syncId": "b4dee7dc-4f15-48ca-a282-63e2c04e0689" // Some unique id
    }
  ]
  ```

  ```json Monitoring theme={null}
  [
    {
      "name": "Feature 'Year' is not quasi-constant",
      "description": "Asserts that the feature 'Year' does not have a very low variance",
      "type": "integrity",
      "subtype": "quasiConstantFeature",
      "thresholds": [
        {
          "insightName": "featureProfile",
          "insightParameters": [
              {"name": "name", "value": "Year"} // Selects feature `Year`
          ],
          "measurement": "isQuasiConstant",
          "operator": "is",
          "value": False
        }
      ],
      "subpopulationFilters": null,
      "mode": "monitoring",
      "usesProductionData": true,
      "evaluationWindow": 3600, // 1 hour
      "delayWindow": 0,
      "syncId": "b4dee7dc-4f15-48ca-a282-63e2c04e0689" // Some unique id
    }
  ]
  ```
</CodeGroup>

## Related

* [Quasi-constant feature count](/tests/integrity/quasi-constant-feature-count).