The terms X1X2 and X 2i (i = 1, 2 or 3) represent the interaction and quadratic terms, respec- tively [21]. The dependent and independent variables select- ed are shown in Table 1 along with their low, medium and high levels, which were selected based on the results from preliminary experimentation. The amounts of HPMC K15M (X1), HPMC K100M (X2) and Na-CMC (X3) used to prepare each of the 15 formulations and the observed responses are given in Table 2.