  We present a study of the acceleration efficiency of suprathermal electrons
at collisionless shock waves driven by interplanetary coronal mass ejections
(ICMEs), with the data analysis from both the spacecraft observations and
test-particle simulations. The observations are from the 3DP/EESA instrument
onboard \emph{Wind} during the 74 shock events listed in Yang et al. 2019, ApJ,
and the test-particle simulations are carried out through 315 cases with
different shock parameters. It is shown that a large shock-normal angle,
upstream Alfv$\acute{\text e}$n Mach number, and shock compression ratio would
enhance the shock acceleration efficiency. In addition, we develop a
theoretical model of the critical shock normal angle for efficient shock
acceleration by assuming the shock drift acceleration to be efficient. We also
obtain models for the critical values of Mach number and compression ratio with
efficient shock acceleration, based on the suggestion of Drury 1983 about the
average momentum change of particle crossing of shock. It is shown that the
theories have similar trends of the observations and simulations. Therefore,
our results suggest that the shock drift acceleration is efficient in the
electron acceleration by ICME-driven shocks, which confirms the findings of
Yang et al.
