BIOSTATISTICAL APPROACHES TO ASSESSMENT 141Figure 7.2. Three dose-response curves illustrating the versatility of the one-hit model
with adaptive repair.ity qjX + q2x2 < 0, or equivalently, x < -q/q^ The optimal dose in this
case is that value of x that minimizes Px. It is obtained by setting the
derivative of Px equal to zero, solving the resulting equation for x, and
verifying that the second derivative is positive when evaluated at this value
of x.
The multistage model has the advantage of being characterized by a small
number of parameters. These parameters may also be readily estimated
from long-term animal bioassay data. This may also be a disadvantage,
though, because it may be difficult to incorporate pharmacokinetic knowl
edge into the estimation process. Nevertheless, in the absence of additional
biological data, the multistage model will often be the default choice in
model-fitting because of its simplicity and parsimony.