Both S(1) and the BIP or set of BIPs must be categorical. This model integrates over the estimated distribution of S(1) | BIP
integrate_nonparametric(formula, ...)
Formula specifying the integration model for the surrogate under treatment. Generally the candidate surrogate will be on the left side in the formula, and the BIP or BIPs will be on the right side. In this case the BIP and the S(1) must be categorical.
Not currently used