  We analyse electron acceleration by a large-scale electric field $E$ in a
collisional hydrogen plasma under the solar flare coronal conditions based on
approaches proposed by Dreicer and Spitzer for the dynamic friction force of
electrons. The Dreicer electric field $E_{Dr}$ is determined as a critical
electric field at which the entire electron population runs away. Two regimes
of strong ($E \lesssim E_{Dr}$) and weak ($E \ll E_{Dr}$) electric field are
discussed. It is shown that the commonly used formal definition of the Dreicer
field leads to an overestimation of its value by about five times. The critical
velocity at which the electrons of the ``tail'' of the Maxwell distribution
become runaway under the action of the sub-Dreiser electric fields turns out to
be underestimated by $\sqrt{3}$ times in some works because the Coulomb
collisions between runaway and thermal electrons are not taken into account.
The electron acceleration by sub-Dreicer electric fields generated in the solar
corona faces difficulties.
