I need to calculate the average time of an event, and I'd like to calculate standard deviation as well. The problem is - there is no such thing as negative time - zero is a boundary - so how does it make sense if the average is 4000us (microseconds) and the stdev is 5000us? Is there a different measure that I should be using when there is a boundary on one-side, like some sort of a one-directional standard deviation? "Semi-infinite interval" comes to mind, and I Wikipedia'ed such probability distributions for more information, but I'm not sure what to make of it. Thanks in advance!