# OffsetTerm


An offset term, e.g. `offset(log_exposure)`.


Usage

``` python
OffsetTerm(expression)
```


An offset is a covariate whose coefficient is fixed at exactly `1` rather than estimated -- it is added directly onto the linear predictor unpenalized and unscaled. The canonical use case is modeling rates with a count-based family: for a Poisson model of event counts with varying exposure times, `offset(log(exposure))` lets the model fit the rate per unit exposure while accounting for the fact that longer exposures mechanically produce more events (since `log(mu) = eta + log(exposure)` implies `mu / exposure = exp(eta)`, the rate).

The `expression` attribute stores the raw, unparsed text found inside `offset(...)` in the formula string (e.g. `"log(exposure)"` or `"log_exposure"`) -- expression evaluation, if any, happens later when the model matrix is built.


## Parameters


`expression: str`  
Raw expression text inside `offset(...)`.