EzyFit Function Reference | << Prev | Next >> |

Fit data with arbitrary fitting function

**ezfit**(FUN) fits the active curve with the function FUN. See below for
the syntax of FUN. If FUN is not specified, 'linear' is used.
By default, the first curve in the active figure is fitted - see
fitparam to change this default behavior. To fit another curve, select
it before calling **ezfit**. If some data are selected by the "Data
Brushing" tool (only for Matlab >= 7.6), only those data are fitted.
**ezfit**(X,Y,FUN) or **ezfit**(Y,FUN) fits the data (X,Y) (or Y) using the
function FUN. X and Y must be vectors of equal length. If X is not
specified, X=[1, 2, 3...] is assumed.
**ezfit**(X,Y,FUN), where X is a 1-by-N vector and Y is a 2-by-N matrix,
also specifies the weights for Y(2,:) (error bars). By default, when
Y is a 1-by-N vector, all the weights are 1.
Note that **ezfit** only computes the coefficients, but does not display the
fit. Use showfit to display the fit.
The function string FUN can be:
- the name of a predefined fitting function (see below).
- the name of a user-defined fitting function (see editfit).
- an equation, in the form 'y(x)=...', where 'x' represents the
X-data, and all the other variables are parameters to be fitted
('a', 'x_0', 'tau', ...). Example: 'y(x)=a*sin(b*x)'. If the
left-hand-side 'y(x)' is not specified, 'x' is taken for the
X-Data. All the parameter names are accepted, except Matlab
reserved strings ('sin', 'pi', ...)
The predefined fitting functions are:
- linear y = m * x
- affine or poly1 y = a*x + b
- poly{n} y = a0 + a1 * x + ... + an * x^n
- power y = c*x^n
- sin y = a * sin (b * x)
- cos y = a * cos (b * x)
- exp y = a * exp (b * x)
- log y = a * log (b * x)
- cngauss y = exp(-x^2/(2*s^2))/(2*pi*s^2)^(1/2)
- cfgauss y = a*exp(-x^2/(2*s^2))
- ngauss y = exp(-(x-x0)^2/(2*s^2))/(2*pi*s^2)^(1/2)
- gauss y = a*exp(-(x-x0)^2/(2*s^2))
'ngauss' is a 2-parameters normalized Gaussian, and 'gauss' is a
3-parameters non-normalized (free) Gaussian. 'cngauss' and 'cfgauss'
are centered normalized and centered free Gaussian, respectively.
**ezfit** is based on Matlab's built-in FMINSEARCH function (Nelder-Mead
method), which performs an unconstrained nonlinear minimization of
the SSR (sum of squared residuals) with respect to the various
parameters.
The correlation coefficient R is defined as SSreg / SStot, where
SSreg = sum ((y_fit - mean(y)).^2) % regression sum of squares
SStot = sum ((y - mean(y)).^2) % total sum of squares
(see https://en.wikipedia.org/wiki/Pearson_product-moment_correlation_coefficient)
(NB: definition of R changed in Ezyfit v2.44)
Nonlinear minimization requires starting guesses (or starting estimates)
for the fit parameters. By default, all the starting guesses are taken
as 1, or, when using predefined fits (e.g., exp, gauss, power...), the
starting guesses are determined depending on the range of the data to
be fitted. However, in most cases, values closer to the expected
result should be specified to "help" the convergence. It is sufficient
to choose values that have the correct sign and correct order of
magnitude, e.g. 0.01, 1, 100...
The starting guesses for the parameters may be specified in two ways:
- directly in the string FUN, after the fit definition:
'c0 + a*sin(pi*x/lambda); c0=1; a=0.1; lambda=100'
('!' or '$' may also be used instead of ';').
- by specifying them as an additional input argument for **ezfit**:
**ezfit**(x,y,'c0 + a*sin(pi*x/lambda)',[0.1 1 100]);
Note that in this case the parameters must be ordered alphabetically.
Note that if both methods are used, only the starting guesses given in
the string FUN are considered.
By default, Y is fitted in linear mode. If you want to fit LOG(Y)
instead, you must specify the option 'log' to the string FUN, separated
by the symbol ';' or '$' or '!' (eg, FUN='a*x^n;log'). This is
specially useful to fit power laws with equally weighted points in a
log scale. If nothing specified, the option 'lin' is used.

**Example**
plotsample('power')
and compare the results of:
**ezfit**('power;lin')
**ezfit**('power;log')
F = **ezfit**(...) also returns a structure F having the following fields:
- name name of the fit
- eq equation of the fit
- param cell array of strings: names of the parameters
- m values of the coefficients
- m0 initial guess for the coefficients
- r correlation coefficient R (Pearson's correlation)
- fitmode 'lin' (y is fitted) or 'log' (log(y) is fitted) mode
This structure F can be further used with showfit, dispeqfit,
showeqbox, makevarfit and editcoeff.
From F, you can get the values of the fitted parameters. If you want to
create in the current Matlab workspace the variables associated to
these parameters, use makevarfit (or set the option 'automakevarfit'
to 'on' in fitparam).

**Examples**
plotsample damposc
f = **ezfit**('u(t) = c + u_a * sin(2*pi*t/T) * exp(-t/tau); T=5; tau=20');
showfit(f);
plotsample poly2
[x,y] = pickdata;
f = **ezfit**(x, y, 'z(v) = poly3');
editcoeff(f);
plotsample poly2
f = **ezfit**('beta(z) = poly2');
showfit(f, 'fitcolor', 'red', 'fitlinewidth', 2);

**See Also**
showfit, plotsample, dispeqfit, editcoeff,
FMINSEARCH, makevarfit, fitparam.
Published output in the Help browser
showdemo **ezfit**

Previous: ezfft | Next: fitparam |

2005-2016 EzyFit Toolbox 2.44