We investigate whether the merger announcement dates provided in the Securities Data Corporation
(SDC) database are handled correctly by researchers performing event studies. We find
that in 24.1% of deals, the popular choice of using the SDC’s “Date Announced” field as the
event date leads to biased estimates of target firm abnormal returns because of earlier abnormal
price movements due to merger-related events such as merger rumors or search-for-buyer types
of announcements. We hand collect the merger-related events from news sources and make the
complete data set publicly available at the Financial Management website.