**3.1 Experimental designs used during FbD of oral DDS**

An experimental design serves as the basis of the FbD exercise. In systematic FbD optimization of DDS, a thorough "screening" of crucial variables is followed by a study of the experimental design-based response surface. Out of all experimental designs, oral DDS has been widely optimized using factororial and central composite designs [12–17]. The main experimental approaches used for oral DDS optimization are compared in **Table 3**, along with their advantages and disadvantages. Full factorial designs (FDs), including two-level and three-level FDs, fractional factorial designs (CCD), Box-Behnken designs (BBD), Plackett-Burman designs (PBD), Taguchi methods, and mixture designs, are among the several types of experimental designs (**Figure 3**) [19].
