> For the complete documentation index, see [llms.txt](https://docs.rootcause.ai/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.rootcause.ai/more-details/applications/application-modules/forecastbacktest.md).

# Forecast Backtest

> Score past forecasts against what actually happened — accuracy over time.

Scores a ForecastSim in this application against what actually happened since it last ran.

It reads the forecasts the named module already produced — `lookbackRuns` of them — and joins them to actuals, so accuracy comes from the runs that really happened rather than a re-simulation. Durable and long-running, because it walks a history of runs.

## At a glance

|                          |                        |
| ------------------------ | ---------------------- |
| Module type              | `ForecastBacktest`     |
| Category                 | Intelligence           |
| Default tier             | `durable`              |
| Runs on                  | `reactive`, `durable`  |
| Starts a flow            | no                     |
| Holds a body             | no                     |
| Keeps state between runs | no                     |
| Available in             | every application type |

## Parameters

### `forecastModuleId`

`string` · required · defaults to `""`

The ForecastSim in this application whose past runs are scored.

### `lookbackRuns`

`number` · required · defaults to `12`

How many of that module's runs to go back over.

### `horizons`

`number[]` · optional

1-based lead times to score; omit for adaptive defaults from forecastH + cadence.

### `primaryHorizon`

`number` · optional

KPI / trend horizon; must be in `horizons` (or adaptive list).

### `metrics`

`string[]` · optional · defaults to `["mape","wmape","mae","mse","rmse","bias","coverage"]`

Metrics to compute (mape, wmape, mae, mse, rmse, bias, coverage).

### `primaryMetric`

`string` · optional · defaults to `"mape"`

KPI / trend metric; must be in `metrics`.

## Inputs

| Port | Kind    | Required | Description                                                                                                                           |
| ---- | ------- | -------- | ------------------------------------------------------------------------------------------------------------------------------------- |
| `on` | `event` | no       | Implicit on every module that does not itself start a flow: wire an event here to make this module the start of what that event runs. |

## Outputs

| Port         | Kind        | Connects to           | Description                                                                                  |
| ------------ | ----------- | --------------------- | -------------------------------------------------------------------------------------------- |
| `comparison` | `dataframe` | `dataframe`, `signal` | One row per forecast point beside the actual it is now scored against.                       |
| `metrics`    | `dataframe` | `dataframe`, `signal` | One row per scored run: every metric in `metrics`, for the run as a whole.                   |
| `byHorizon`  | `dataframe` | `dataframe`, `signal` | The same metrics split by lead time, so a 1-month error is not averaged with a 12-month one. |
| `summary`    | `dataframe` | `dataframe`, `signal` | The latest run's accuracy alone, for a KPI tile.                                             |

***

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