  The experimental observations of sequential double ionization (SDI) of Ar [A.
N. Pfeiffer {\it et al.}, Nature Phys. {\bf 7}, 428 (2011)], such as the
four-peak momentum distribution and the ionization time of the first and second
electrons, are investigated and reproduced with a quantum model by including
and excluding the $e$-$e$ correlation effect. Based on the comparison of
experiment and simulation, the role of $e$-$e$ correlation in SDI is discussed.
It is shown that the inclusion of $e$-$e$ correlation is necessary to reproduce
the momentum distribution of electrons.
