See Linear regression . The easiest way to fit a curve is to create a cubic spline or lowess curve. They are easier than nonlinear regression, because you don't have to choose an equation. Spline and lowess curves tend to wiggle too much, so are not often used as standard curves. See Fitting a curve without choosing a model .
Polynomial regression is a convenient method to create a smooth curve.
With Prism, you perform polynomial regression by choosing a polynomial equation from the nonlinear regression dialog. Try a second, third or fourth order polynomial equation. The higher order polynomial equations generate standard curves with more inflection points. See Polynomial Regression . These assays are based on competitive binding.