X-Stream Operator’s Manual
WM-OM-E Rev I 153
pctl Percentile
Definition:
Computes the horizontal data value that separates the data in a histogram such
that the population on the left is a specified percentage `xx' of the total
population. When the threshold is set to 50%, pctl is the same as hmedian.
Description:
The total population of the histogram is determined. Scanning from left to right,
the population of each bin is summed until a bin that causes the sum to equal or
exceed `xx'% of the population value is encountered. A ratio of the number of
counts needed for `xx'% population/total bin population is then determined for the
bin. The horizontal value of the bin at that ratio point of its range is found, and
returned as pctl.
Example:
The total population of a histogram is 100. The histogram range is divided into 20
bins and `xx' is set to 25%. The population sum at the sixth bin from the left is 22.
The population of the seventh is 9 and its sub-range is 6.1 to 6.4 V. The ratio of
counts needed for 25% population to total bin population is:
3 counts needed / 9 counts = 1/3.
The value for pctl is:
6.1 volts + .33 * (6.4 6.1) volts = 6.2 volts.
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