Basic goodness-of-fit plots for the final piperaquine model. Observations plotted against population predicted concentrations (A) and against individual predicted concentrations (B). Conditional weighted residuals plotted against population predicted concentrations (C) and time (D). The solid line is the identity line and the broken line is the locally weighted least square regression line. The concentrations were transformed into their logarithms (base 10). Conditional weighted residuals were fixed between 4 and −4, excluding one data point.