Percentile in all its forms is doing a count of the points in your viz - in your example there are 8 - it then counts the points and divides them in 2 groups based on the "Value" - in the .5 example that equates to 4 points above the returned value and 4 points below - for .75 it is 2 points above and 6 below (25% and 75% of the points )
When the boundary falls between 2 point Percentile will do an interpolation to come up with a value -
Does that help
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So, for the 75th percentile.. First tableau gets the 75 % by (total number) *.75 gives us the value of the index when we see is value on that index ?
it is based on Marks for the filtered view- it then returns a value of the MEASURE that equates to the boundary -
so if the number of points is 100 and if the sum of sales is $100 for the value 75th and $90 for the 76th Percentile will return $95 half way between the 2 values
NOT the index