Monterey Bay Aquarium Research Institute
Edward T. Peltzer
Model I and Model II Regressions

Which Regression: Model I or Model II?

  • For Excel®, MATLAB® and most other commercial programs the inherent line fitting method is the model-I regression.
  • To determine whether you are using a model-I or a model-II regression,
    • first find the slope of Y vs X where Y is plotted on the vertical axis and X is plotted on the horiziontal axis -- this is the "normal" way of doing things. It is also known as the regression of Y-on-X. Call this slope m(y).
    • Now reverse X and Y and fit another line; call this slope m(x)'.
    • Since X and Y are reversed, we need to find the inverse of m(x)' to properly compare against m(y),
      so let m(x) = 1 / m(x)'.
    • Now if m(x) = m(y) exactly and r is not equal to 1, then you are using a model-II regression.
    • If m(x) is not equal to m(y), then you are using a model-1 regression.
  • Note that for either model, r^2 = m(y) / m(x). This is known as the Pearson product-moment correlation coefficient. It is a measure of the linearity of the data, not the fit of the line to the data.
  • To quickly calculate the model-II geometric mean regression slope, m(gm), first determine the model-I regression slope, m(y), and the correlation coefficient, r. The geometric mean slope is then calculated as: m(gm) = m(y) / r. Or, you can use the MATLAB® script file lsqfitgm.
  • Also note that for datasets where r = 1, m(y) = m(x) = m(gm). In those cases, this test will not tell you which method you are using.





Last updated: Feb. 05, 2009
Questions? Comments? Please contact Edward Peltzer.