The bimodal nature of these observed distributions can be reasonably well explained by considering that real assays tend to have a limit in how low the activity they detect can be characterized. This may be caused by practical constraints (e.g. compound solubility) or simply that we're not interested in characterising degrees of inactivity (activity levels that have no commercial interest). The effect of applying this sort of censoring to a normal distribution is shown below (figure ), with values below zero set to zero and a 2 normal distribution fitted.