  The CKMT model for the nucleon structure function F_2 is in good agreement
with the HERA data at low and moderate Q^2. The fit to the same data obtained
with a modified version of the model in which a logarithmic dependence on Q^2
has been included is also discussed. Finally, we show how the parametrization
of the CKMT model for the nucleon structure function F_2 describes the HERA
data when presented in the Caldwell-plot as the behavior of the logarithmic
slopes of F_2 vs x and Q^2.
