Uncertanity analysis using cut-off or Threshold value



Dear members,
I have an experimentally measured data points(y) for the variable x. These data points are random data and more than 100. I have plotted these points in a graph (x,y) and find the correlation between them. It was non-linear second order polynomial.
I have a cut-off value in the experimentally measured data points. I want to do uncertainty analysis or in other words I want to find out the uncertainty range in the variable x using the cut-off value.

What is the best statistical method could be adopted to find out the uncertainty range

Thank you for your time

Thank you