Have you made a common error when using nonlinear regression?

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s	Questions
	Are results sensible?
	SE and CI
	Goodness of fit
	Systematic deviation
	Local minimum
	Assumptions
	Common errors

Comparing two curves

Distributions of best-fit values

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Have you made a common error when using nonlinear regression?

Consider these possibilities if you obtained an error message rather than results, or if the nonlinear regression results made no sense.

Potential problem	Solution
The equation simply does not describe the data.	Try a different equation.
The initial values are too far from their correct values.	Enter different initial values. If you entered your own equation, check the rules for initial values. See .
The range of X values is too narrow to define the curve completely.	If possible, collect more data. Otherwise, hold one of the variables to a constant value.
You have not collected enough data in a critical range of X values.	Collect more data in the important regions.
Your data are very scattered and don't really define a curve.	Try to collect less scattered data. If you are combining several experiments, normalize the data for each experiment to an internal control.
The equation includes more than one component, but your data don't follow a multicomponent model.	Use a simpler equation.
Your numbers are too large.	If your X or Y values are huge, change the units. Avoid values greater than about 10000.
Your numbers are too small.	If your X or Y values are tiny, change the units. Avoid values less than about 0.001.
You've set a parameter to an inappropriate constant value.	Either don't set the parameter to a constant value, or change that value. Perhaps you made a simple mistake like setting a maximum plateau to 1.0 when it should be 100, or a Hill slope to +1.0 when it should be -1.0.