# 3.118 FAQ-691 How to Find X/Y from Fitted Curves in Find X/Y table?

Last Update: 3/3/2015

The Find X/Y table allows you to obtain either a dependent variable value given an independent variable value, or an independent variable value given a dependent variable value, from the fit you performed on the data.

For example, in Linear Fit dialog, There are check boxes in FindX/Y node which turn on FindX/Y output and allows calculation of 95% LCL and UCL values After Checking on the checkbox, there will be a new worksheet contained Find X/Y table named something like *FindXfromY* (or *FindYfromX*). You can enter the Y (or X) values into the first column in the table and the X (or Y) values and 95% LCL and UCL values will be calculated automatically. In the Nonlinear curve fit tool (NLFit), you can specify extra X/Y columns using a drop-down list. If there are multiple solutions, those values will be added in the extra columns. For example, in the model, there might be two x values for a given y value. So you might specify the Number of X Columns as 2 in the Find X from Y branch.

Generally, Find Y from X runs faster than Find X from Y. When finding Y from X, Origin uses the fit parameter values directly in the fitting model, to calculate the Y values. However, if you choose to find X from Y, Origin will not be able to derive an X~Y equation automatically. It has to calculate the approximate values by iteration. The algorithm used is illustrated below:

Origin first creates a uniform linear curve. For a given Y value, the range containing the Y value is found. For example, the Y value of a given point might fall into the range [yn, yn+1]. We know the X value should be within the range [xn, xn+1]. Then a new Y value y', which corresponds to X = (xn + xn+1) / 2, is computed. With the computed y', we can divide the range [yn, yn+1] into two subranges [yn, y'] and [y', yn+1]. Then y and y' are compared to see in which subrange y is located. These steps are repeated until the difference between y and y' is within some tolerance $1e^{-8}$.

Note that the iteration method is used in Find X from Y. You can only find the X values for Y values within the source data range.

Keywords:Find Y from X, Error Analysis, Linear Curve Fit