diff --git a/latan/FitInterface.cpp b/latan/FitInterface.cpp
new file mode 100644
index 0000000..f11dec5
--- /dev/null
+++ b/latan/FitInterface.cpp
@@ -0,0 +1,178 @@
+/*
+ * FitInterface.cpp, part of LatAnalyze 3
+ *
+ * Copyright (C) 2013 - 2014 Antonin Portelli
+ *
+ * LatAnalyze 3 is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * LatAnalyze 3 is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with LatAnalyze 3.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <latan/FitInterface.hpp>
+#include <latan/includes.hpp>
+
+using namespace std;
+using namespace Latan;
+
+/******************************************************************************
+ *                       FitInterface implementation                          *
+ ******************************************************************************/
+// constructors ////////////////////////////////////////////////////////////////
+FitInterface::FitInterface(const Index nData, const Index xDim,
+                           const Index yDim)
+{
+    resize(nData, xDim, yDim);
+}
+
+// access //////////////////////////////////////////////////////////////////////
+void FitInterface::assumeXExact(const Index i, const bool isExact)
+{
+    isXExact_(i) = (isExact) ? 1 : 0;
+}
+
+void  FitInterface::assumeXXCorrelated(const Index i1, const Index i2,
+                                       const bool isCorrelated)
+{
+    isXXCorr_(i1, i2) = (isCorrelated) ? 1 : 0;
+}
+
+void  FitInterface::assumeYYCorrelated(const Index j1, const Index j2,
+                                       const bool isCorrelated)
+{
+    isYYCorr_(j1, j2) = (isCorrelated) ? 1 : 0;
+}
+
+void  FitInterface::assumeYXCorrelated(const Index j, const Index i,
+                                       const bool isCorrelated)
+{
+    isYXCorr_(j, i) = (isCorrelated) ? 1 : 0;
+}
+
+void  FitInterface::assumeDataCorrelated(const Index k1, const Index k2,
+                                         const bool isCorrelated)
+{
+    isDataCorr_(k1, k2) = (isCorrelated) ? 1 : 0;
+}
+
+void FitInterface::fitPoint(const Index i, const bool isFitPoint)
+{
+    isFitPoint_(i) = (isFitPoint) ? 1 : 0;
+}
+
+void FitInterface::fitPointRange(const Index k1, const Index k2,
+                                 const bool isFitPoint)
+{
+    int size = static_cast<int>(k2-k1+1);
+    
+    isFitPoint_.segment(k1, size) = IVec::Constant(size, (isFitPoint) ? 1 : 0);
+}
+
+void FitInterface::fitAllPoints(const bool isFitPoint)
+{
+    fitPointRange(0, getNData()-1, isFitPoint);
+}
+
+Index FitInterface::getNData(void) const
+{
+    return isFitPoint_.size();
+}
+
+Index FitInterface::getNFitPoint(void) const
+{
+    return isFitPoint_.sum();
+}
+
+Index FitInterface::getXDim(void) const
+{
+    return isXXCorr_.rows();
+}
+
+Index FitInterface::getYDim(void) const
+{
+    return isYYCorr_.rows();
+}
+
+Index FitInterface::getStatXDim(void) const
+{
+    return isXExact_.size() - isXExact_.sum();
+}
+
+void FitInterface::setFitInterface(const FitInterface &fitInterface)
+{
+    if (&fitInterface != this)
+    {
+        resize(fitInterface.getNData(), fitInterface.getXDim(),
+               fitInterface.getYDim());
+        isXExact_   = fitInterface.isXExact_;
+        isFitPoint_ = fitInterface.isFitPoint_;
+        isXXCorr_   = fitInterface.isXXCorr_;
+        isYYCorr_   = fitInterface.isYYCorr_;
+        isYXCorr_   = fitInterface.isYXCorr_;
+        isDataCorr_ = fitInterface.isDataCorr_;
+    }
+}
+
+void FitInterface::setNData(const Index nData)
+{
+    resize(nData, getXDim(), getYDim());
+}
+
+void FitInterface::setXDim(const Index xDim)
+{
+    resize(getNData(), xDim, getYDim());
+}
+
+void FitInterface::setYDim(const Index yDim)
+{
+    resize(getNData(), getXDim(), yDim);
+}
+
+void FitInterface::resize(const Index nData, const Index xDim, const Index yDim)
+{
+    isXExact_.setConstant(xDim, 0);
+    isFitPoint_.setConstant(nData, 0);
+    isXXCorr_.setIdentity(xDim, xDim);
+    isYYCorr_.setIdentity(xDim, xDim);
+    isYXCorr_.setConstant(yDim, xDim, 0);
+    isDataCorr_.setIdentity(nData, nData);
+}
+
+// test ////////////////////////////////////////////////////////////////////////
+bool FitInterface::isFitPoint(const Index k) const
+{
+    return (isFitPoint_[k] == 1);
+}
+
+bool FitInterface::isXExact(const Index i) const
+{
+    return (isXExact_[i] == 1);
+}
+
+bool FitInterface::isXXCorrelated(const Index i1, const Index i2) const
+{
+    return (isXXCorr_(i1, i2) == 1);
+}
+
+bool FitInterface::isYYCorrelated(const Index j1, const Index j2) const
+{
+    return (isYYCorr_(j1, j2) == 1);
+}
+
+bool FitInterface::isYXCorrelated(const Index j, const Index i) const
+{
+    return (isYXCorr_(j, i) == 1);
+}
+
+bool FitInterface::isDataCorrelated(const Index k1, const Index k2) const
+{
+    return (isDataCorr_(k1, k2) == 1);
+}
diff --git a/latan/FitInterface.hpp b/latan/FitInterface.hpp
new file mode 100644
index 0000000..c746c26
--- /dev/null
+++ b/latan/FitInterface.hpp
@@ -0,0 +1,79 @@
+/*
+ * FitInterface.hpp, part of LatAnalyze 3
+ *
+ * Copyright (C) 2013 - 2014 Antonin Portelli
+ *
+ * LatAnalyze 3 is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * LatAnalyze 3 is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with LatAnalyze 3.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef Latan_FitInterface_hpp_
+#define Latan_FitInterface_hpp_
+
+#include <latan/Global.hpp>
+#include <latan/Minimizer.hpp>
+
+BEGIN_NAMESPACE
+
+/******************************************************************************
+ *                         base class for data fit                            *
+ ******************************************************************************/
+class FitInterface
+{
+public:
+    typedef Minimizer::Verbosity FitVerbosity;
+public:
+    // constructors
+    FitInterface(void) = default;
+    FitInterface(const Index nData, const Index xDim, const Index yDim);
+    // destructor
+    virtual ~FitInterface(void) = default;
+    // access
+    void  assumeXExact(const Index i, const bool isExact = true);
+    void  assumeXXCorrelated(const Index i1, const Index i2,
+                             const bool isCorrelated = true);
+    void  assumeYYCorrelated(const Index j1, const Index j2,
+                             const bool isCorrelated = true);
+    void  assumeYXCorrelated(const Index j, const Index i,
+                             const bool isCorrelated = true);
+    void  assumeDataCorrelated(const Index k1, const Index k2,
+                               const bool isCorrelated = true);
+    void  fitPoint(const Index k, const bool isFitPoint = true);
+    void  fitPointRange(const Index k1, const Index k2,
+                        const bool isFitPoint = true);
+    void  fitAllPoints(const bool isFitPoint = true);
+    Index getNData(void)     const;
+    Index getNFitPoint(void) const;
+    Index getXDim(void)      const;
+    Index getYDim(void)      const;
+    Index getStatXDim(void)  const;
+    void  setFitInterface(const FitInterface &fitInterface);
+    void  setNData(const Index nData);
+    void  setXDim(const Index xDim);
+    void  setYDim(const Index yDim);
+    void  resize(const Index nData, const Index xDim, const Index yDim);
+    // test
+    bool isFitPoint(const Index k) const;
+    bool isXExact(const Index i) const;
+    bool isXXCorrelated(const Index i1, const Index i2) const;
+    bool isYYCorrelated(const Index j1, const Index j2) const;
+    bool isYXCorrelated(const Index j, const Index i) const;
+    bool isDataCorrelated(const Index k1, const Index k2) const;
+private:
+    IVec isXExact_, isFitPoint_;
+    IMat isXXCorr_, isYYCorr_, isYXCorr_, isDataCorr_;
+};
+
+END_NAMESPACE
+
+#endif // Latan_FitInterface_hpp_
diff --git a/latan/Makefile.am b/latan/Makefile.am
index ec97a26..2813509 100644
--- a/latan/Makefile.am
+++ b/latan/Makefile.am
@@ -35,6 +35,7 @@ liblatan_la_SOURCES =   \
     Global.cpp          \
     includes.hpp        \
     File.cpp            \
+    FitInterface.cpp    \
     Mat.cpp             \
     Math.cpp            \
     MathInterpreter.cpp \
@@ -53,6 +54,7 @@ liblatan_la_HEADERS =   \
     Chi2Function.hpp    \
     CompiledFunction.hpp\
     Dataset.hpp         \
+    FitInterface.hpp    \
     Function.hpp        \
     Global.hpp          \
     File.hpp            \
diff --git a/latan/XYStatData.cpp b/latan/XYStatData.cpp
index 35b8de5..7ac54d1 100644
--- a/latan/XYStatData.cpp
+++ b/latan/XYStatData.cpp
@@ -32,6 +32,11 @@ double FitResult::getChi2(void) const
     return chi2_;
 }
 
+Index FitResult::getNDof(void) const
+{
+    return nDof_;
+}
+
 double FitResult::getChi2PerDof(void) const
 {
     return chi2_/static_cast<double>(nDof_);
@@ -57,69 +62,6 @@ XYStatData::XYStatData(const Index nData, const Index xDim, const Index yDim)
 }
 
 // access //////////////////////////////////////////////////////////////////////
-void XYStatData::assumeXExact(const Index i, const bool isExact)
-{
-    isXExact_[i] = (isExact) ? 1 : 0;
-}
-
-void XYStatData::fitPoint(const Index i, const bool isFitPoint)
-{
-    isFitPoint_[i] = (isFitPoint) ? 1 : 0;
-}
-
-void XYStatData::fitPointRange(const Index k1, const Index k2,
-                               const bool isFitPoint)
-{
-    int size = static_cast<int>(k2-k1+1);
-    
-    isFitPoint_.segment(k1, size) = IVec::Constant(size, (isFitPoint) ? 1 : 0);
-}
-
-void XYStatData::fitAllPoints(const bool isFitPoint)
-{
-    fitPointRange(0, getNData()-1, isFitPoint);
-}
-
-Index XYStatData::getNData(void) const
-{
-    return x_.rows();
-}
-
-Index XYStatData::getNFitPoint(void) const
-{
-    return isFitPoint_.sum();
-}
-
-Index XYStatData::getXDim(void) const
-{
-    return x_.cols();
-}
-
-Index XYStatData::getYDim(void) const
-{
-    return y_.cols();
-}
-
-Index XYStatData::getStatXDim(void) const
-{
-    return isXExact_.size() - isXExact_.sum();
-}
-
-void XYStatData::setNData(const Index nData)
-{
-    resize(nData, getXDim(), getYDim());
-}
-
-void XYStatData::setXDim(const Index xDim)
-{
-    resize(getNData(), xDim, getYDim());
-}
-
-void XYStatData::setYDim(const Index yDim)
-{
-    resize(getNData(), getXDim(), yDim);
-}
-
 void XYStatData::resize(const Index nData, const Index xDim, const Index yDim)
 {
     x_.conservativeResize(nData, xDim);
@@ -263,17 +205,6 @@ ConstBlock<DMatBase> XYStatData::yxVar(const Index j, const Index i) const
     return FULL_BLOCK(var_[yx](j, i));
 }
 
-// test ////////////////////////////////////////////////////////////////////////
-bool XYStatData::isFitPoint(const Index k) const
-{
-    return (isFitPoint_[k] == 1);
-}
-
-bool XYStatData::isXExact(const Index i) const
-{
-    return (isXExact_[i] == 1);
-}
-
 // fit /////////////////////////////////////////////////////////////////////////
 FitResult XYStatData::fit(const vector<const DoubleModel *> &modelVector,
                           Minimizer &minimizer, const DVec &init,
diff --git a/latan/XYStatData.hpp b/latan/XYStatData.hpp
index e2da320..84c70a3 100644
--- a/latan/XYStatData.hpp
+++ b/latan/XYStatData.hpp
@@ -22,6 +22,7 @@
 
 #include <latan/Global.hpp>
 #include <latan/Chi2Function.hpp>
+#include <latan/FitInterface.hpp>
 #include <latan/Function.hpp>
 #include <latan/Mat.hpp>
 #include <latan/Minimizer.hpp>
@@ -44,6 +45,7 @@ public:
     virtual ~FitResult(void) = default;
     // access
     double                 getChi2(void) const;
+    Index                  getNDof(void) const;
     double                 getChi2PerDof(void) const;
     const DoubleFunction & getModel(const Index j = 0) const;
 private:
@@ -64,7 +66,6 @@ public:
         yy = 1,
         yx = 2
     };
-    typedef Minimizer::Verbosity FitVerbosity;
 public:
     // constructors
     XYStatData(void);
@@ -72,19 +73,6 @@ public:
     // destructor
     virtual ~XYStatData(void) = default;
     // access
-    void                 assumeXExact(const Index i, const bool isExact = true);
-    void                 fitPoint(const Index k, const bool isFitPoint = true);
-    void                 fitPointRange(const Index k1, const Index k2,
-                                       const bool isFitPoint = true);
-    void                 fitAllPoints(const bool isFitPoint = true);
-    Index                getNData(void)     const;
-    Index                getNFitPoint(void) const;
-    Index                getXDim(void)      const;
-    Index                getYDim(void)      const;
-    Index                getStatXDim(void)  const;
-    void                 setNData(const Index nData);
-    void                 setXDim(const Index xDim);
-    void                 setYDim(const Index yDim);
     void                 resize(const Index nData, const Index xDim,
                                 const Index yDim);
     Block<DMatBase>      x(const PlaceHolder ph1, const PlaceHolder ph2);
@@ -109,9 +97,6 @@ public:
     ConstBlock<DMatBase> yyVar(const Index j1, const Index j2) const;
     Block<DMatBase>      yxVar(const Index j,  const Index i);
     ConstBlock<DMatBase> yxVar(const Index j,  const Index i) const;
-    // test
-    bool isFitPoint(const Index k) const;
-    bool isXExact(const Index i) const;
     // fit
     FitResult fit(const std::vector<const DoubleModel *> &modelVector,
                   Minimizer &minimizer, const DVec &init,
@@ -120,6 +105,7 @@ public:
     FitResult fit(const DoubleModel &model, Minimizer &minimizer,
                   const DVec &init, const bool reinitChi2 = true,
                   const FitVerbosity verbosity = FitVerbosity::Silent);
+                  
 private:
     DMat         x_, y_;
     Mat<DMat>    var_[3];