# Metric differences between platforms

Platforms do not measure the same things. Onekey maps every platform onto one [unified schema](https://onekey.dev/docs/schema.md), and where a platform has no equivalent of a field it says so instead of inventing one.

## The rule

- **A fixed-schema field means one thing everywhere.** `author.followers` is a follower count on every platform that emits it; `engagement.likes` is a like count. If a platform's number is something else (a "reach", a "score", a "karma"), it does not go in the shared field.
- **No substitution.** If a platform does not expose a value, the field is `null` when the endpoint maps it and absent when it does not. A different number is never put in its place. See [Honest nulls](https://onekey.dev/docs/data-availability.md#honest-nulls).
- **Near-equivalents live under `ext`.** A platform-specific metric that is close to, but not the same as, a shared one is returned under `ext.<name>` with its own name, documented on the endpoint page. You choose whether it is good enough for your purpose.

## How to see what a given endpoint returns

1. Open the endpoint page and read **Field mapping**: it lists every field the endpoint emits.
2. Look at the **Recorded response**. A mapped field with no value for that record is `null`.
3. Call `GET /v1/platforms` for the same information in JSON.

## Computed values

The [computed block](https://onekey.dev/docs/computed.md) adds `engagement_rate`, `estimated_reach` and `language` on many endpoints. They are arithmetic or detection over fields already in the answer, with the rule published and a warning when an input is missing, so the value is `null` rather than a guess when the inputs are not there.

## Practical advice

- For cross-platform code read only fixed-schema fields, and treat `null` as unknown.
- Do not compare a raw count across platforms without checking what the platform counts (a view on one platform may be a play or an impression on another). The platform's own definition is the one that holds.
- If you need a metric a platform does not expose, it is not available from public data. Say so in your product rather than estimating it.

## Next

- [Data availability](https://onekey.dev/docs/data-availability.md).
- [Unified schema](https://onekey.dev/docs/schema.md).