# Can I back fit or evaluate a point on a standard curve fit?**Document ID:** DQ308
**Relevant to following products:** WorkOut 2.5 Manta 2.5
**Document description:** Can I back fit or evaluate a point on a standard curve fit?
In many situations it is useful to be able to back fit or evaluate a point on a standard curve fit. For example, you may want to find what the response is for a given concentration on the fit. The "Standard Curve Fit" transform performs this for you automatically on you samples, however you might want to find points on the fit from other data sources.
With R3 and later you can determine the X value for a specified Y value on a curve and vice versa.
For example, to calculate what the Y value is on a fit for a given X value of 10, assuming the first transform is a "Standard Curve Fit" transform you can write the expression:
#1=CalcY(10)
Similarly to calculate the X value for a Y response of 10% of the Control1 wells use:
#1=CalcX(Control1*0.1)
These expression can be used in any transform which accepts an expression. For finding a single value use the expression in the "Expression" transform.
It is important to note that the number following the # symbol is the number of the "Standard Curve Fit transform" (the first being 1). The transform has to exist prior to the transform in which the expression is defined.
Another point to note, when evaluating points in a fit which may have multiple results (such as finding X for a given Y in a polynomial 3rd order expression), since only one result can be returned by the expression, the first results is always given.
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