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Curve fit in MATLAB

  1. Dec 10, 2007 #1
    I have a set of data and I need to fit a curve to it. The equation of the curve is:
    My problem is just getting a curve fit to this equation. What function in MATLAB should I be looking for? How do I get the value of 'a'?

    Is it easier to achieve this in Excel? How?
  2. jcsd
  3. Dec 10, 2007 #2


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    Plot it, and then look in the menus in the plot window for a fitting tool.

    All of the functions available in that menu are also available on the command line, but I find curve fitting to be much more intuitive when done in a plot window.

    - Warren
  4. Dec 11, 2007 #3
    will i be able to enter in my own custom equation? thanks
  5. Dec 13, 2007 #4
    In excel if you have all the points plotted out, it will curve fit the points to whatever order of polynomial you want.

    In Matlab I am sure you can enter your custom equations.
  6. Dec 13, 2007 #5


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    Yes, you can fit to whatever function you want.
    Note that if you use functions from the Optimization toolbox (if you have that installed) you can quite easily fit to a nonlinear Matlab function, i.e. you can "fit" to a piece of code which generates an output that depends on an number of inputs, even if the function includes e.g. interpolation (which is often used conventient when fitting to a large set of experimental data), if-then statements etc. This is often very useful.

    Also, if you are able to rewrite your equations as a matrix equation (e.g. a sum of exponentials with unknown prefactors) you can of course also solve it directly, i.e. using something like "X\b" which gives you the best result in the least-square sense.
  7. Mar 14, 2008 #6
    Fit in matlab

    You can also try Ezyfit, a free curve fitting toolbox for Matlab:
    In your case just type
    if your data are plotted in a figure, or
    f = ezfit(x,y,'y=20-a*10*log(x)')
    if your data are stored into the vectors x,y.
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