  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. A total of seven energy channels ranging from 0.428
to 4.161 keV are selected. In the simulations, using a backward-in-time method,
we calculate the average downstream flux in the $90^\circ$ pitch angle. On the
other hand, the average downstream and upstream fluxes in the $90^\circ$ pitch
angle can also be directly obtained from the 74 observational shock events. In
addition, the variation of the event number ratio with downstream to upstream
flux ratio above a threshold value in terms of the shock angle (the angle
between the shock normal and upstream magnetic field), upstream
Alfv$\acute{\text e}$n Mach number, and shock compression ratio is
statistically obtained. It is shown from both the observations and simulations
that a large shock angle, upstream Alfv$\acute{\text e}$n Mach number, and
shock compression ratio can enhance the shock acceleration efficiency. Our
results suggest that shock drift acceleration is more efficient in the electron
acceleration by ICME-driven shocks, which confirms the findings of Yang et al.
2018.
