I have observed too many funds that have steadily moved Up for an extended period of time, only to suddenly loose 33% or more of their total value in a single day.

To avoid this situation, the hourly 30 day Standard Deviation is computed for each Sector.
If there is sudden and major downdraft in a well defined trending Sector, I will sell it whenever the the percentage drop exceeds two times it's standard deviation.

As an example I sold Automotive and Home-Finance the hour their large percentage drop was posted.
Please refer to the Home-Finance chart.