Topic $1$: In event studies, how do we compute the $($statistical$)$ significance of abnormal returns? you can describe in either math sentence or verbal statement. Why is it important?

Topic $2$: In event studies, to measure abnormal returns, we use the actual return during event to subtract the returns calculated from index model $($either one or two factor$).$ Why don't we just use average returns in past year as benchmark instead of factor model based ones? which one is more accurate, and why?