Edward T. Peltzer
Model I and Model II Regressions

## MATLAB® shell-scripts for linear regression analysis

• Model I regressions
• lsqfity - Standard Model I linear regression
• [Y-on-X regression]
• Fit line by minimizing y-residuals only
• All data points are given equal weight
• Use whenever X is controlled and Y is measured
• lsqfitx - Alternate or Reversed Model I regression
• [X-on-Y regression]
• Fit line by minimizing x-residuals only
• All data points are given equal weight
• lsqfityw - Weighted Model I regression
• [wY-on-X regression]
• Fit line by minimizing y-residuals only
• Data points are given varying weight
• Use whenever X is controlled and Y is measured with varying uncertainty
• Please note that:
• This m-file gives the correct results for all parameters (Bevington and Robinson,1992); but,
• the uncertainties in the slope and intercept are much smaller than the same uncertainties calculated by lsqfity.m or lsqfitxi.m (see the results table).
• lsqfityz - Weighted Model I regression - revised Jan 2000
• [zY-on-X regression]
• This is the same regression as lsqfityw.m, except that:
• York's (1966) equations for the uncertainties in weighted slope and intercept are used.
• The revised uncertainties are more are more in line with those from lsqfity.m.
• Model II regressions
• lsqfitma - Pearson's Major Axis
• aka: first principal component
• [Correlation of X & Y]
• Line is fit by minimizing BOTH x- and y-residuals simultaneously
• All data are given equal weight
• Use when units and range of X and Y are the same
• lsqfitgm - Geometric Mean regression
• aka: reduced major axis or standard major axis
• [Correlation of X & Y]
• Slope of line is the geometric mean of the two slopes determined by regressing Y-on-X and X-on-Y
• All data are given equal weight
• Use when units or range of X and Y are different
• Please note that:
• This algorithm uses both lsqfity.m and lsqfitx.m for determining the slope.
• The uncertainties in the slope and intercept are estimated by analogy with the symmetrical uncertainty limits for a model I regression following the treatment by Ricker (1973).
• lsqbisec - Least Squares Bisector
• [Regression/correlation of X & Y]
• Slope of line is determined by bisecting the minor angle between the two model I regressions:
Y-on-X and X-on-Y
• All data are given equal weight
• Use when units or range of X and Y are different
• Please note that:
• This algorithm uses both lsqfity.m and lsqfitx.m for determining the slope.
• The uncertainties in the slope and intercept are estimated by analogy with the symmetrical uncertainty limits for a model I regression following the treatment by Ricker (1973).
• lsqcubic - Least-Squares-Cubic
• [Regression/correlation of wX & wY.]
• Line is fit by minimizing both x- and y-residuals simultaneously for WEIGHTED data points.
• Each data point can be given its own weight either as the inverse-square of the actual measurement precision or as the inverse-square of the product of the relative measurement precision for the method times concentration.
• Use when the measurement error of X and/or Y varies.
• Please note that:
• This algorithm uses lsqfitma.m for the first estimate of the slope.
• Iteration proceeds until the change in the slope is less than the user defined limit.