Basic-program-usage.md

@@ -29,6 +29,21 @@ Inclusion of the statistical or systematic correlations of the data in the fit i
 ```
  in this case the statistical correlations for H1_NormInclJets_HighQ2_99-07 data file are included (the method also allows to include correlations between data sets via file names, i.e: H1_NormInclJets_HighQ2_99-07___H1_InclJets_HighQ2_99-00.dat.corr)
 
+As additional option for data sets with covariance matrix, it is possible to convert covariance matrix to nuisance parameter representation (following the prescription suggested by J. Gao and P. Nadolsky in arXiv:1401.0013):
+
+```
+&CovarToNuisance 
+  ! Global switch for using nuisance param representation for covariance mat. 
+  LConvertCovToNui = .true. 
+  ! Tolerance -- zero means exact transformation 
+  Tolerance = 0.0 
+  ! The following lines allow to adjust error scaling properties     
+  ! (default: :M - multiplicative, A - additive) 
+  DataName = 'CMS electon Asymmetry rapidity', 'CMS W muon asymmetry' 
+  DataSystType = ':A', ':A' 
+&End
+```
+
 The results of the minimization are printed to the standard output and written to the files in the `output/` directory (name of the directory can be changed in the steering.txt). The quality of the fit can be judged based on total $`\chi^2`$ per degrees of freedom. It is printed for each iteration as
 
 The resulting $`\chi^2`$ is reported for each data set and for correlated systematic uncertainties separately. This information is printed and written to the `output/Results.txt` file. The `Results.txt` file contains additional information about shifts of the correlated systematic uncertainties.