Nonlinear Curve Fitting in Origin (6.1)
1. Copy and paste data into an Origin data sheet
2. Select both columns and plot the data (click appropriate plot button)
3. Choose "analysis" and "nonlinear curve fit"
a. Choose "function"
i. Choose "select" if the function exists
ii. Choose "edit" if a function is close, and you want to make some
changes
iii. Choose "new" if you want to input a function
1. Give the function a name
2. Choose the number of fitting parameters
3. Carefully type in the function using the correct parameter
names:
4. y = P1*exp(3*P2*x)-sin(P3*x^2)
5. note: you can use user-defined names if you check the box
6. Choose "y-script" in the "form" drop-down box at bottom
7. Click "save"
b. Choose "action" and "fit"
i. Enter guesses for the parameter values
1. Do not use zero
ii. You can choose to fit or to fix a parameter ("vary?") - if you have a
lot of parameters, you might want to try fitting some while keeping
some fixed, especially if you have some good guesses for values.
iii. Hit the 1 Iter or 10 Iter button. This makes it go through 1 or 10
iterations of trying different parameters and measuring ChiSquared. Hit it as many times as you want subject to the next
comment.
1. The idea behind non-linear fitting is that you want to
minimize the Chi-Squared value. Chi-Squared is the
square-root of the sum of the squares of the distances of
each data point from the theoretical curve (a measure of the
distance from the data to the curve). Watch the Chi-Square
value to see when it no longer changes (after you hit an
"Iter" button) - at that point, the program cannot fit any
better with the current parameter values.
2. Each time you hit an "Iter" button, it will plot the new fit on
the graph in place of the old one.
iv. If the fit does not look good on the graph, you will need to
manually change some parameter values, or play with fitting only
one or two at a time.
v. Choose "done" and it will print the parameter values on the graph.