diff --git a/source/include/Ostap/ValueWithError.h b/source/include/Ostap/ValueWithError.h
index 6f0647c6..5efca8bc 100644
--- a/source/include/Ostap/ValueWithError.h
+++ b/source/include/Ostap/ValueWithError.h
@@ -1517,11 +1517,37 @@ namespace Ostap
     ( ValueWithError& left , 
       ValueWithError& right ) { left.swap ( right ) ; }
     // ========================================================================
-    // converison to string
+    // conversion to string
     inline std::string to_string 
     ( const ValueWithError& v ) 
     { return v.toString() ; }
     // ========================================================================
+    /** Calculate mean andRMS  for two samples 
+     *  @param  a   mean and  rms for the 1st sample
+     *  @param na   size of the first sample  
+     *  @param  b   mean and  rms for the 2nd sample
+     *  @param nb  size of the second sample  
+     *  @return mean and rmd for the combined sample 
+     */
+    ValueWithError two_samples
+    ( const ValueWithError& a  ,
+      const unsigned long   na ,
+      const ValueWithError& b  ,
+      const unsigned long   nb ) ;
+    // ========================================================================
+    /** Calculate mean andRMS  for two samples 
+     *  @param  a   mean and  rms for the 1st sample
+     *  @param na   size of the first sample  
+     *  @param  b   mean and  rms for the 2nd sample
+     *  @param nb  size of the second sample  
+     *  @return mean and rmd for the combined sample 
+     */
+    ValueWithError two_samples
+    ( const ValueWithError& a  ,
+      const double          na ,
+      const ValueWithError& b  ,
+      const double          nb ) ;
+    // ========================================================================
   } //                                         The end of namespace Ostap::Math 
   // ==========================================================================
 } //                                                 The end of namespace Ostap
diff --git a/source/src/ValueWithError.cpp b/source/src/ValueWithError.cpp
index beb224c9..ff18f7f5 100644
--- a/source/src/ValueWithError.cpp
+++ b/source/src/ValueWithError.cpp
@@ -3280,6 +3280,93 @@ Ostap::Math::agm
   //
   return agm ( x , y , 0.0 ) ;
 }
+// =============================================================================
+namespace
+{
+  // ===========================================================================
+  inline
+  Ostap::Math::ValueWithError
+  _two_samples_ 
+  ( const Ostap::Math::ValueWithError& a      ,
+    const Ostap::Math::ValueWithError& b      ,
+    const long double                  alpha  )
+  {
+    const double  beta  = 1.0L - alpha ;
+    //
+    const double am  = a.value() ;
+    const double bm  = b.value() ;
+    const double ac2 = std::max ( a.cov2() , 0.0 ) ;
+    const double bc2 = std::max ( b.cov2() , 0.0 ) ;
+    //
+    const double m   = alpha * am + beta * bm ;
+    const double dm  = am - bm ;
+    const double c2  = alpha * ac2  + beta * bc2 + alpha * beta * dm * dm ;
+    //
+    return Ostap::Math::ValueWithError ( m , c2 ) ;
+  }
+  // ===========================================================================
+}
+// =============================================================================
+/* Calculate mean and RMS  for two samples 
+ *  @param  a   mean and  rms for the 1st sample
+ *  @param na   size of the first sample  
+ *  @param  b   mean and  rms for the 2nd sample
+ *  @param nb  size of the second sample  
+ *  @return mean and rmd for the combined sample 
+ */
+// =============================================================================
+Ostap::Math::ValueWithError
+Ostap::Math::two_samples
+( const Ostap::Math::ValueWithError& a  ,
+  const unsigned long                na ,
+  const Ostap::Math::ValueWithError& b  ,
+  const unsigned long                nb )
+{
+  const unsigned long nt = na + nb ;
+  //
+  static const std::string s_m1 { "Invalid sample size"      } ;
+  static const std::string s_m2 { "Ostap::Math::two_samples" } ;
+  //
+  Ostap::Assert ( 0 < nt , s_m1 , s_m2 ) ;
+  //
+  if      ( !na  ) { return b ; }
+  else if ( !nb  ) { return a ; }
+  //
+  const long double la = na ;
+  //
+  return _two_samples_ ( a , b , la / nt ) ;
+}
+// =============================================================================
+/* Calculate mean and RMS  for two samples 
+ *  @param  a   mean and  rms for the 1st sample
+ *  @param na   size of the first sample  
+ *  @param  b   mean and  rms for the 2nd sample
+ *  @param nb   size of the second sample  
+ *  @return mean and rmd for the combined sample 
+ */
+// =============================================================================
+Ostap::Math::ValueWithError
+Ostap::Math::two_samples
+( const Ostap::Math::ValueWithError& a  ,
+  const double                       na ,
+  const Ostap::Math::ValueWithError& b  ,
+  const double                       nb )
+{
+  const double nt = na + nb ;
+  //
+  static const std::string s_m1 { "Invalid sample size"      } ;
+  static const std::string s_m2 { "Ostap::Math::two_samples" } ;
+  //
+  Ostap::Assert ( 0 < nt , s_m1 , s_m2 ) ;
+  //
+  if      ( !na  ) { return b ; }
+  else if ( !nb  ) { return a ; }
+  //
+  const long double la = na ;
+  //
+  return _two_samples_ ( a , b , la / nt ) ;
+}
+// =============================================================================