The REGRESS function performs a multiple linear regression fit and returns an *Nterm*-element column vector of coefficients.

REGRESS fits the function:

*y _{i}* =

*const*+

*a*

_{0}

*x*

_{0,}

*+*

_{i}*a*

_{1}

*x*

_{1,}

*+ ... +*

_{i}*a*

_{Nterms}_{-1}

*x*

_{Nterms}_{-1,}

_{i}This routine is written in the IDL language. Its source code can be found in the file regress.pro in the lib subdirectory of the IDL distribution.

## Examples

; Create two vectors of independent variable data:

X1 = [1.0, 2.0, 4.0, 8.0, 16.0, 32.0]

X2 = [0.0, 1.0, 2.0, 3.0, 4.0, 5.0]

; Combine into a 2x6 array

X = [TRANSPOSE(X1), TRANSPOSE(X2)]

; Create a vector of dependent variable data:

Y = 5 + 3*X1 - 4*X2

; Assume Gaussian measurement errors for each point:

measure_errors = REPLICATE(0.5, N_ELEMENTS(Y))

; Compute the fit, and print the results:

result = REGRESS(X, Y, SIGMA=sigma, CONST=const, $

MEASURE_ERRORS=measure_errors)

PRINT, 'Constant: ', const

PRINT, 'Coefficients: ', result[*]

PRINT, 'Standard errors: ', sigma

IDL prints:

`Constant: 4.99999`

Coefficients: 3.00000 -3.99999

Standard errors: 0.0444831 0.282038

## Syntax

*Result* = REGRESS( *X*, *Y*, [, CHISQ=*variable*] [, CONST=*variable*] [, CORRELATION=*variable*] [, /DOUBLE] [, FTEST=*variable*] [, MCORRELATION=*variable*] [, MEASURE_ERRORS=*vector*] [, SIGMA=*variable*] [, STATUS=*variable*] [, YFIT=*variable*] )

## Return Value

REGRESS returns a 1 x *Nterm* array of coefficients. If the DOUBLE keyword is set, or if *X* or *Y* are double-precision, then the result will be double precision, otherwise the result will be single precision.

## Arguments

### X

An *Nterms* by *Npoints* array of independent variable data, where *Nterms* is the number of coefficients (independent variables) and *Npoints* is the number of samples.

### Y

An *Npoints*-element vector of dependent variable points.

## Keywords

### CHISQ

Set this keyword equal to a named variable that will contain the value of the unreduced chi-square goodness-of-fit statistic.

### CONST

Set this keyword to a named variable that will contain the constant term of the fit.

### CORRELATION

Set this keyword to a named variable that will contain the vector of linear correlation coefficients.

### DOUBLE

Set this keyword to force computations to be done in double-precision arithmetic.

### FTEST

Set this keyword to a named variable that will contain the F-value for the goodness-of-fit test.

### MCORRELATION

Set this keyword to a named variable that will contain the multiple linear correlation coefficient.

### MEASURE_ERRORS

Set this keyword to a vector containing standard measurement errors for each point *Y*[*i*]. This vector must be the same length as *X* and *Y*.

**Note: **For Gaussian errors (e.g., instrumental uncertainties), MEASURE_ERRORS should be set to the standard deviations of each point in *Y*. For Poisson or statistical weighting, MEASURE_ERRORS should be set to SQRT(*Y*).

### SIGMA

Set this keyword to a named variable that will contain the 1-sigma uncertainty estimates for the returned parameters.

**Note: **If MEASURE_ERRORS is omitted, then you are assuming that the regression model is the correct model for your data, and therefore, no independent goodness-of-fit test is possible. In this case, the values returned in SIGMA are multiplied by SQRT(CHISQ/(*N*–*M*)), where *N* is the number of points in *X*, and *M* is the number of coefficients. See section 15.2 of *Numerical Recipes in C* (Second Edition) for details.

### STATUS

Set this keyword to a named variable that will contain the status of the operation. Possible status values are:

- 0 = successful completion
- 1 = singular array (which indicates that the inversion is invalid)
- 2 = warning that a small pivot element was used and that significant accuracy was probably lost.

**Note: **If STATUS is not specified, any error messages will be output to the screen.

### YFIT

Set this keyword to a named variable that will contain the vector of calculated *Y* values.

## Version History

Original |
Introduced |

5.4 |
Deprecated the |

## See Also

CURVEFIT, GAUSSFIT, LINFIT, LMFIT, POLY_FIT, SFIT, SVDFIT