Inc. All Rights Reserved. It does not attempt to find a better equation. Prism doesn't have to fit all the parameters in a model. In many cases, it is appropriate to set one or more parameters to constant values. See Fixing parameters to constant values .
Nonlinear regression assumes that the scatter is Gaussian.
Nonlinear regression assumes that the scatter of points around the best-fit curve has the same standard deviation all along the curve. Y values also tend to be further from the best-fit curve. The assumption that the standard deviation is the same everywhere is termed homoscedasticity . The nonlinear regression model assumes that X values are exactly correct, and that experimental error or biological variability only affects the Y values. The deviation of each value from the curve should be random, and should not be correlated with the deviation of the previous or next point.