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The default is 5 minutes and that could explain the output you are seeing.-d: delta time
You can put the delta time as an option : the "delta" is the prefered time between two values that the script will use to calculate the average Kbytes/s or error/min. The delta time should (not must) be bigger than the check interval.
Here is an example : Check interval of 2 minutes and delta of 4min
T0 : value 1 : can't calculate usage
T0+2 : value 2 : can't calculate usage
T0+4 : value 3 : usage=(value3-value1)/((T0+4)-T0)
T0+6 : value 4 : usage=(value4-value2)/((T0+6)-T0+2)
(Yes I know TO+4-T0=4, it's just to explain..)
.........
The script will allow 10% less of the delta and 300% more than delta as a correct interval.
For example, with a delta of 5 minutes, the acceptable interval will be between 4'30" and 15 minutes.