The surfaces obtained for DHICP, DHICapryol and STDCP were complex, and the removal of a single point completely changed the mathematical model, justifying the low R 2 predicted values. However, since the other parameters were adequate, the models obtained were considered representative of these samples. Table shows the values of the bi coefficients of the models. The fit parameters and residuals for the DHICP are shown in Figure , as an example. The residuals were expected to be random, and thus, to follow a normal distribution, as shown in Figure ; homoscedastic, i.e., the same variance over the range of predicted values was observed (Figure ) and independent (Figure ). Finally, the prediction capability of the model was evaluated from the plot of experimental vs predicted values (Figure ). This parameter indicates if the model is able to predict well the experimental response. Similar results were obtained for the other responses DHICapryol, STDCP and STDCapryol (data not shown). Figure shows the contour maps for each response. In all cases, the region of higher BTB concentration showed lower STDhistograms and DHI values (blue region in the lower right of the maps). However, as seen in the chemical images (Figure ) these regions were homogeneous due to the drug going to the surface, and thus, they represent a region of unsuitable proportions. DHICP and DHICapryol contour maps had similar information: the region of highest heterogeneity (in red) was that with the highest concentrations of Capryol ® 90, while the most homogeneous regions were those with highest concentrations of cetyl palmitate and lowest API and liquid excipient concentrations. The maps constructed based on the STD of the histograms showed different information: while for the solid lipid (cetyl palmitate) the intermediary concentration/region was the most heterogeneous, for the liquid lipid the most heterogeneous region was that near to binary mixtures of CP and Capryol ® 90 (orange to red in Figure ). This is an indication that STD is not a suitable parameter in this case. In this sense, the maximum API that could be used in this NLC is less than 5.0% (w/v).