Dichotomous (quantal) dose-response models having alternative background parameters were externally reviewed by experts in June 2007. These models were included as part of the BMDS 2.0 beta. Reviews were generally positive and confimed that the functioning of the computer code had been rigorously tested. The reviewers' comments, EPA's responses, and the model development and testing report are available for review. 

The new models modified existing BMDS quantal models. They were developed by modifying existing source codes, changing the code only as necessary. Adding these new models to the BMDS model suite expanded the range of alternatives available for fitting dose-response curves using generally accepted types of models.

At the time, BMDS's dichotomous models had a background term that was directly additive to the response probability. In this case, five new models implemented an alternative form of background term, one that is additive to the dose: multistage, cancer, log-probit, gamma, and Weibull.

Models with background terms additive to dose were proposed and analysed in peer-reviewed papers by leading experts over 20 years ago, so they are not new to risk analysis. One type of model, log-logistic, was not provided in a version with background additive to dose, because of technical difficulties causing unreliable convergence on solutions.

Two models -- probit and logistic -- that previously had no explicit background response parameter, became available in that form.

Therefore, with the exception of the log-logistic, all the BMDS dichotomous models available in BMDS 2.0 beta had two versions, differing only in the background term. The new models kept the names used in BMDS 1.4.1c. New versions with an alternative background parameter were distinctively named to identify them clearly, e.g., "Weibull-BgDose" or "Probit-BgResp."