Number and spacing of doses for a dose-finding trial. The doses in Fig. 2(a) are too low to estimate the MTD, whereas those in Fig. 2(b) are too high. In Fig. 2(c), the target dose lies between two dose levels, so patients will be assigned alternately to an overdose level and an underdose level; the final MTD will likely be at one of these levels. Figure 2(d) illustrates a situation with several dose levels available in the region of the MTD.