“…These constraints can be product related or process related and are either one or two sided inequality constraints that are easily represented in mathematical terms. A pivotal moment in the advancement of QbD was reached when practitioners recognized that it was possible to mathematically defined a design space as a subspace of the knowledge space, where the estimated probability for a system to fulfill all given constraints is greater than a minimum acceptable risk. − This can be written as follows: where DS, design space (a subset of x ); f , the model of the system; g , the set of constraints; h , the probability function for each element of x to fulfill the constraints ( g ); x , all possible combinations of process parameters in the n dimensional knowledge space (K n ); y , model predictions (presumably quality attributes of interest); π, minimum acceptable risk; σ , expected common cause variability; Θ , set of model parameters; and Σ Θ , variance covariance of model parameters…”