###
Keywords

**NOTE: **
One of the following keywords specifying a type of model must be set when using COMFIT. If you do not specify a model, IDL will display a warning message when COMFIT is called.

####
EXPONENTIAL

Set this keyword to compute the parameters of the exponential model.

####
GEOMETRIC

Set this keyword to compute the parameters of the geometric model.

####
GOMPERTZ

Set this keyword to compute the parameters of the Gompertz model.

####
HYPERBOLIC

Set this keyword to compute the parameters of the hyperbolic model.

####
LOGISTIC

Set this keyword to compute the parameters of the logistic model.

####
LOGSQUARE

Set this keyword to compute the parameters of the logsquare model.

####
SIGMA

Set this keyword to a named variable that will contain a vector of standard deviations for the computed model parameters.

####
WEIGHTS

Set this keyword equal to a vector of weights for *
Y*
_{
i}
. This vector should be the same length as *
X*
and *
Y*
. The error for each term is weighted by WEIGHTS_{
i}
when computing the fit. Frequently, WEIGHTS_{
i}
= 1.0/s^{
2}
_{
i}
, where s is the measurement error or standard deviation of *
Y*
_{
i}
(Gaussian or instrumental weighting), or WEIGHTS = 1/Y (Poisson or statistical weighting). If WEIGHTS is not specified, WEIGHTS_{
i}
is assumed to be 1.0.

####
YFIT

Set this keyword to a named variable that will contain an *
n*
-element vector of y-data corresponding to the computed model parameters.

###
Example

Define two *
n*
-element vectors of paired data.

X = [ 2.27, 15.01, 34.74, 36.01, 43.65, 50.02, 53.84, 58.30, $

62.12, 64.66, 71.66, 79.94, 85.67, 114.95]

Y = [ 5.16, 22.63, 34.36, 34.92, 37.98, 40.22, 41.46, 42.81, $

43.91, 44.62, 46.44, 48.43, 49.70, 55.31]

Define a 3-element vector of initial estimates for the logsquare model.

A = [1.5, 1.5, 1.5]

Compute the model parameters of the logsquare model, A[0], A[1], & A[2].

result = COMFIT(X, Y, A, /LOGSQUARE)

The result should be the 3-element vector: [1.42494, 7.21900, 9.18794].