unction <code><a href='https://rdrr.io/r/base/rank.html'>rank</a></code> with
parameter <code>ties.method = "average"</code> and <code>na.last = "keep"</code>. For
this test the estimation period and the event period must not overlap,
otherwise an error will be thrown. The test statistic is assumed to have a
normal distribution (as an approximation). The test is well-specified for the
case, when cross-sectional abnormal returns are not symmetric. The test is
stable to variance increase during event window. 