Before revision:   In particular, we have detected two sudden jumps in the standard deviation, in the second quarter of 2017 and at the beginning of 2018, suggesting early warning signals of two majors price collapse that have happened in those periods.
After revision:  In particular, we have detected two sudden jumps in the standard deviation, in the second quarter of 2017 and at the beginning of 2018, which could have served as early warning signals of two majors price collapse that have happened in those periods.