# Customizing the density estimator¶

`sbi`

allows to specify a custom density estimator for each of the implemented methods. For all options, check the API reference here.

## Changing the type of density estimator¶

One option is to use one of set of preconfigured density estimators by passing a string in the `density_estimator`

keyword argument to the inference object (`SNPE`

or `SNLE`

), e.g., “maf” to use a Masked Autoregressive Flow, of “nsf” to use a Neural Spline Flow with default hyperparameters.

```
inference = SNPE(prior=prior, density_estimator="maf")
```

In the case of `SNRE`

, the argument is called `classifier`

:

```
inference = SNRE(prior=prior, classifier="resnet")
```

## Changing hyperparameters of density estimators¶

Alternatively, you can use a set of utils functions to configure a density estimator yourself, e.g., use a MAF with hyperparameters chosen for your problem at hand.

Here, because we want to use SN*P*E, we specifiy a neural network targeting the *posterior* (using the utils function `posterior_nn`

). In this example, we will create a neural spline flow (`'nsf'`

) with `60`

hidden units and `3`

transform layers:

```
from sbi.utils.get_nn_models import (
posterior_nn,
) # For SNLE: likelihood_nn(). For SNRE: classifier_nn()
density_estimator_build_fun = posterior_nn(
model="nsf", hidden_features=60, num_transforms=3
)
inference = SNPE(prior=prior, density_estimator=density_estimator_build_fun)
```

It is also possible to pass an `embedding_net`

to `posterior_nn()`

which learn summary statistics from high-dimensional simulation outputs. You can find a more detailed tutorial on this here.

## Building new density estimators from scratch¶

Finally, it is also possible to implement your own density estimator from scratch, e.g., including embedding nets to preprocess data, or to a density estimator architecture of your choice.

For this, the `density_estimator`

argument needs to be a function that takes `theta`

and `x`

batches as arguments to then construct the density estimator after the first set of simulations was generated. Our utils functions in `sbi/utils/get_nn_models.py`

return such a function.