  We developed a least squares fitter used for extracting expected physics
parameters from the correlated experimental data in high energy physics. This
fitter considers the correlations among the observables and handles the
nonlinearity using linearization during the $\chi^2$ minimization. This method
can naturally be extended to the analysis with external inputs. By
incorporating with Lagrange multipliers, the fitter includes constraints among
the measured observables and the parameters of interest. We applied this fitter
to the study of the $D^{0}-\bar{D}^{0}$ mixing parameters as the test-bed based
on MC simulation. The test results show that the fitter gives unbiased
estimators with correct uncertainties and the approach is credible.
