Declaration: 
TCurveFit.PercentileX (prob: double): double; 
The function PercentileX returns the percentile of the distribution of all xvalues entered by EnterStatValue. The parameter prob specifies the probability in percent (valid range: 0..100). If prob is set to a value outside the valid range it is automatically restricted to the closest boundary of the valid range.

Hint: 
The calculated percentile is not interpolated and simply returns the closest value of the distribution, which may differ from results obtained from statistical packages if the number of included values is low. 
